Skip to main content

Full text of "Cassell's reinforced concrete; a complete treatise on the practice and theory of modern construction in concrete-steel"

See other formats 




aSotS'KJiSJl V- 



CASSELL'S REINFORCED CONCRETE 



Cassell's 



Reinforced Concrete 

A Complete Treatise on the Practice and 

Theory of Modern Construction 

in Concrete-Steel 

Edited by 

Bernard E. Jones 

Editor of " Building World " 

Assisted by 

Albert Lakeman 

M.C.I., Honours Medallist in Building Construction 

and by a Staff of Specialist Writers 



Illustrated by 171 Photographs and about Five Hundred 
Diagrams and Working Drawings 






Cassell and Company, Limited 
London, New York, Toronto and Melbourne 

1913 






List of Chief Contributors and Revisers 

FRANK B. GATEHOUSE, F.C.S Portland Cement 

THOMAS POTTER, M.G.I Concrete 

A. B. SEARLE Concrete, Cement, Waterproofing, etc. 

E. L. RHEAD, M.Sc., Tech. F.I.G Steel 

ALBERT LAKEMAN, M.G.I Theory, Examples, etc. 

Honours Medallist in Construction 

V. SUSSEX HYDE, M.G.I. . Practical Erection, Forms and Centerings 

F. CHARLES BARKER, M.S.A Examples of Forms 

Assist. Dangerous Structure Surveyor, City of London 

T. ELSON HARDY Architectural Treatment 

A. SEYMOUR JENNINGS, F.I.B.D. . . ... Painting Concrete 

W. H. BROWN, F.S.I Estimating, Quantities, etc. 

THE EDITOR .... Introduction, History, Examples, etc. 



267323 



Preface 

THE purpose of this book is to provide a practical, simply-worded guide to construc- 
tion in reinforced concrete, as distinct from the theoretical and often obscure treatises 
that have appeared in large numbers during the past few years. The theoretical side 
of the subject has not, however, been overlooked, as any treatise on reinforced con- 
crete that ignored methods of calculating the various members would be obviously 
incomplete, especially as this method of construction offers great possibilities of 
scientific design. As a matter of fact, the two theoretical chapters are a strong feature 
of the book, having been written, at my request, by Mr. Albert Lakeman, who has 
brought a trained mind to bear upon his subject, and, in addition to explaining impor- 
tant principles in a simple and concise manner and clearly showing the practical 
application of the theories he expounds, has imparted to his text the unusual element 
of freshness. Mr. Lakeman has wisely begun at the beginning, and, whilst he could 
not finish at the end for one thing, the end is not yet he will be found to take an 
apt and diligent student far enough on the road for him to be able to find the rest 
of the way for himself ; and, after all, no book can do more than that for anyone. 

The book is largely concerned with the practical side of reinforced concrete 
with what has been done and can be done in the compound material, and how to do 
it. The introductory chapter having explained the advantages of reinforced con- 
crete, an historical chapter (every definite date in which I have verified from official 
records) puts the reader in possession of a variety of facts, many of which will throw 
light on the subject as his knowledge of it grows. Concrete and steel, as materials, 
are discussed in the two following chapters, in the earlier of which special attention 
is devoted to portland cement and the modern methods of mixing and handling 
concrete. Then come the explanations of theory, followed by two chapters respec- 
tively devoted to the methods employed in erecting a building and the forms and 
centerings necessitated in general reinforced concrete work ; these two chapters are 
noteworthy, I think, from the practical character of the information they give and 
the very large number of explanatory drawings and photographs which they include. 
Concise descriptions of the chief commercial systems follow, and later chapters deal 
respectively with architectural and surface treatment, durability, waterproofing, 
arches and bridges, and quantity surveying, estimating, measuring and pricing. The 
concluding chapter describes and illustrates a number of works carried out in re- 
inforced concrete, varying from a palatial club-house and commercial " sky-scraper " 
to railway sleepers and sewer pipes. 

Every endeavour has been made to render the volume as complete as possible, in 
the hope that it will be useful to all classes of readers, both as a text-book and as a 
work of reference. 

I have been happy in having the co-operation of a number of experts (a list of 



viii PREFACE 

whom is given elsewhere). Each of these writers has contributed or revised the section 
relating to the particular phase of the subject he is most competent to deal with ; and 
from many of them I have received valuable suggestions which I have been able to 
apply. I wish to acknowledge, also, the kindly treatment accorded me by a large 
number of firms identified with special systems, materials, machines, etc., for, in a 
number of cases they have supplied me with information, drawings, and photographs. 
Additional thanks are due to Messrs. Geo. West and Co. for permission to make brief 
extracts from the historical notes published in a " Lock-woven Mesh " handbook. 

I owe a special debt of gratitude to the premier concrete journal, Concrete and Con- 
structional Engineering, which has placed many facilities at my disposal and has 
been good enough to lend me a large number of blocks. To the files of that journal, 
too, I have gone for help in many instances when other sources of information 
proved vain. 

The illustrations to this work include no less than 171 photographs and about 
five hundred diagrams and working drawings ; and the large number of " progress " 
photographs, showing the actual practice of reinforced concrete work, constitute, I 
hope, a specially valuable feature of the book, and considerably enhance its teaching 

value. 

B. E. J. 

La Belle Sauvage, 
London, E.G. 



Contents 



PAGE 

Introduction : What Reinforced Concrete Is 1 

The Limitations of Concrete Reinforcement Terminology A Compound 
Material Comparisons. 

Historical Notes . . 5 

Early in the Nineteenth Century A Concrete Boat Wilkinson Frangois 
Coignet Dennett, Allen, Ransome, and Scott Joseph Monier Phillip 
Brannon Thaddeus Hyatt Progress between 1870 and 1892 Edmond 
Coignet and Frangois Hennebique Progress since 1892 Notable American 
Inventions. 

Concrete : Materials, Proportions and Mixing . . . 12 

What Concrete Is Natural Aggregates- Artificial Aggregates Size of Aggregate 
Washing Coarse Aggregate Sand Portland Cement Natural Cements 
Slag Cement Lime Concrete Water for Concrete Mixing Proportioning 
Concrete Gauge Boxes and Measures Mixing Concrete by Hand Machine 
Mixing and Mixers Batch Mixers Described Continuous Mixers Described 
Testing Efficiency of Mixing Conveying Concrete Laying Concrete. 

SteeJ . . 50 

Composition Malleability Ductility Elasticity Limit of Elasticity Testing 
Steel Specification Modulus of Elasticity Certificate of Tests Fatigue 
Toughness Hardness Bending Tests Shearing Strength Resistance to 
Alternating and Repeated Stresses Effects of Heat Co-efficient of Expan- 
sion Other Elements Present in Steel Methods of Manufacture. 

Stress Simply Explained . . 61 

Formulae Principle of Moments Levers Reactions Bending Moments 
Moment of Inertia Section Modulus and Moment of Resistance Calculating 
Safe Load on Beam Columns and Struts Shearing Stress. 

The Theory of Reinforced Concrete 83 

Introductory Data for Calculations Beams with Single Reinforcement Calcu- 
lations for Slabs Beams and Slabs with Double Reinforcement Tee Beams 
Tee Beams Continuous or Fixed at Ends Tee Beams with Double Rein- 
forcement- Shearing Stress and Adhesion Columns Columns Eccentrically 
Loaded Retaining Walls. 

The Erection of a Reinforced Concrete Building . 126 

Building a Factory Tools and Appliances Driven Piles " Compressol " Con- 
crete Piles The Foundation Slabs The Retaining Wall Columns Walls 
Staircases Introducing the Reinforcements into Beam Moulds Forming 
Skeleton Reinforcements Making a Floor. 

Forms and Centerings 

Sizes of Timber Striking the Forms Setting Out Forms for Piles Foundation 
Forms Column, Beam, and Slab Forms Column Forms Centerings for 



x CONTENTS 

PAGE 

Concrete Floors Carried by Steel Joists Wall and Partition Forms Imitating 
Masonry Walls Staircase Forms Metal Forms for Walls, Beams, etc. 
Forms for Cornices, Mouldings, etc. Forms for Ornamental Parapet Forms 
for Retaining Walls Forms for Silos Forms for Domes Forms for Tall 
Chimneys Forms and Centering for Arches and Bridges Suspended Center- 
ing for Bridges Forms for Pipes and Sewers Forms for Tanks Form for 
Rectangular Reservoir Forms for Fence Posts. 

Systems Described . 215 

Armoured Tubular Flooring Clinton System Coignet System Considere System 
Corr Bar Dentile System Expanded Metal System Hennebique System 
Indented Bar System Johnson's Lattice System Kahn System Keedon 
System Lock-woven Mesh System Mushroom System Paragon System 
Piketty System Other Systems. 

Architectural and Surface Treatment of Reinforced Concrete . . 229 

Architectural Treatment Surface Treatment The Untouched Surface Brush 
Finish Carborundum Finish Sand-blast Finish Bush-hammered Finish 
Facing Concrete in the Form P.ebble Dashing Sand Finish Glazed Finish 
Plastered Surfaces Tiles, Mosaic, Sgraffito, etc Stuc Work Colouring Con- 
crete Body Colours Table of Pigments Stains Distempering Painting. 

Durability of Reinforced Concrete 25t> 

Mechanical Destructive Influences Blows and Shocks Earthquakes Settle- 
ments Frost Fire Chemical Destructive Influences Atmosphere Water 
Sea Water Acids Alkalies Urine. 

Waterproofing Concrete 266 

Superficial Waterproofing Mass Waterproofing Making a Non-porous Concrete. 

Specifications, Quantities, Measuring, Estimating, and Pricing 

The Specialist System A General Specification Quantities General Outline 
of Bill or Schedule Designs Independent of the Specialist Measuring Con- 
crete Small Holes Measuring Steel Reinforcement Measuring Centering 
" Use and Waste " First-class Centering " Centering " Clauses in 
Preamble Specially-designed Centering Openings Centering to Walls 
Moulds for Architectural Features Centering to Inclined Slabs, Beams, 
Columns, etc. Abstracting Billing The Preamble Billing Measured Items 
Pricing Prices for Concrete, Steel and Centering. 

Arches and Bridges 

Design of Arches and Bridges Examples of Arches and Bridges. 

Examples 311 

Royal Automobile Club's Building Royal Liver Building Cathedrals at Manila 
and Poti Wesleyan Hall, Westminster Free Church, Hampstead Garden 
City St. Barnabas' Church, Jamaica The Majestic Theatre, Los Angeles 
National Gallery Extension Aluminium Works at Kinlochleven Royal 
Insurance Building, London Wall with Post Buttresses Retaining Wall 
Stairways Chimney Shafts Pipes, Sewers and Conduits Wharf at Denne- 
mont Jetty Head at Thames Haven Lifeboat Slipway- Roofed Reservoir 
Spectators' Stands, etc. Railway Sleepers Grain Silos at Silvertown 
Water Tower at Singen. 



List of Illustrations 



FIG. PAGE 

1 to 3. Columns of Plain Concrete, Steel, and Reinforced Concrete respectively : Comparative 

Diagrams . . . . . . . . . . . . 3 

4 to 6. Beams of Plain Concrete, Steel, and Reinforced Concrete respectively : Comparative 

Diagrams .............. 3 

7. Gauge Box with Fixed Handles . . . . . . . . . . .28 

8. Cement Gauge .............. 28 

9. Sectional Diagram showing Principle of Chicago Cube Mixer . . . . . .31 

10. Chicago Cube Mixer, with Engine and Boiler ......... 31 

11. " Cut-away " View of Eclipse Mixing Drum ......... 32 

12. Eclipse Mixer, with Petrol Motor . ... . . . . . . . .32 

13. " Cut-away " View of Koehring Mixing Drum . . . . . . . .33 

14. Koehring Mixer, with Engine and Boiler ......... 33 

15. Marsh-Capron Non-tilting Mixing Drum . . . . . . . . .33 

16. " Cut-away " View of Marsh-Capron Tilting Mixing Drum . . . . . .34 

17. Marsh-Capron Mixer, with Drum in Tilting Position ....... 34 

18. McKelvey " Gravity Shovel " 35 

19 to 24. Ransome Mixers . . . . . . . . . . . . .36 

25. Scoops in Ransome Mixing Drum (English) . . . . . . ... .36 

26. Scoops in Ransome Mixing Drum (American) . . . . . . . .36 

27. Elevations and Plan of Ransome Belt-driven Mixer. ....... 37 

28. Roll Mixer, with Elevator raised . . . 37 

29. Smith Hand-driven Mixer . . . . . . . . . . . .38 

30. Smith Power-driven Mixer ............ 39 

31. Victoria Mixer, with Skip Elevated .......... 40 

32. Victoria Mixer, with Skip Lowered .......... 40 

33. " Cut-away " View of Victoria Mixing Drum . . . . . . . . . 41 

34. Express Mixer .............. 4i 

35. Fawcett Mixer . ............. 41 

36. Gaspary Hand-driven Tilting Trough Mixer ........ 42 

37. Pansy Mixer ............... 42 

38. Whalley Mixer, with Engine and Boiler .......... 43 

39. Gaspary Drum-type Continuous Mixer .......... 43 

40. Mason Mixer .............. 44 

41. Bolte Mixer ^ .... 44 

41a. Hoppers of Bolte Mixer ............ 44 

42. Goltrin Mixer 45 

43. Coltrin Mixing Blades 45 

44. Kent Mixer ............... 45 

45. Perfect Mixer .............. 45 

46. " Cut-away " View, showing Principle of Trump Mixer ....... 40 

47. Ransome Concrete Cart ............. 46 

48. Ransome Concrete Skip or Bucket .......... 46 

49. Ground Plan, Top Plan, and Two Elevations of Ransome Tower. ..... 47 

50. Graph showing Influence of Carbon on Tenacity of Steel .... .51 

51. Graph showing Influence of Carbon on Ductility of Steel ...... 52 

52. Test Piece before and after Stretching 53 

53 to 55. Photo-micrographs of Steel . . . . . . 58, 59 

56. Lever of First Order 62 

57. Lever of Second Order ............. 62 

58. Lever of Third Order 63 

59. Diagram Showing how a Cantilever Tends to Move. ....... 63 

60. Triangular Portion of Wall Lifted by Movement of Cantilever. ..... 63 

xi 



LIST OF ILLUSTRATIONS 

^G- PAGE 

61. Beam Loaded Eccentrically ........ .64 

62. Diagram Showing how Eccentrically Loaded Beam Tends to Rotate .... 64 

63. Beam Rotating on the other Abutment. ..... .64 

64. Beam Carrying Three Concentrated Loads ... 65 

65. Diagram Illustrating Clear and Effective Spans ..... 65 

66. Centrally-loaded Beam ...... 65 

67. Bending Tendency on Beam ........ 66 

68. Finding Bending Moment of Beam . . . . . . . . . .66 

69. Bending Moment Set Up to Scale ....... .67 

70. Finding Bending Moment of Beam .... 67 

71. Lever Arms of Reaction and Load .......... 67 

72. Beam with Two Loads Concentrated at Different Points ....... 67 

73. Cantilever, with Concentrated Load at Outer End ........ 68 

74. Failure of Cantilever due to Tension ...... 68 

75. Failure of Cantilever due to Compression ......... 68 

76. Bending Moment in Cantilever ....... .68 

77. Bending Moment of Cantilever Set Out to Scale ........ 69 

78. How Centrally-loaded Cantilever Tends to Bend ........ 69 

79. Cantilever with Two Concentrated Loads ......... 69 

80. Bending Tendency in Cantilever with Two Concentrated Loads ..... 70 

81. Beam with Uniformly Distributed Load .......... 70 

82. Distributed Load acting through Centre of Gravity ....... 70 

83. Bending Moment of Beam at Intermediate Point ........ 70 

84. Bending Moment on Beam Set up to Scale . . . . . . . . .71 

85. Beam with Concentrated and Distributed Loading . . . . . . . .71 

86. Cantilever Carrying Distributed Load .......... 71 

87. Finding Bending Moment at Intermediate Point in Cantilever ...... 72 

88. Bending Moment in Cantilever Set Out to Scale ........ 72 

89. Cantilever, with Combined Distributed and Concentrated Loading ..... 72 

90. Calculating Moment of Inertia of Simple Rectangular Section ...... 78 

91 to 97. Common Sections and their Moments of Inertia ....... 74 

98. Calculating Moment of Inertia of Rolled Steel Joist ....... 74 

99. Calculating Least Moment of Inertia of Rolled Steel Joist ...... 75 

100. Rotating Tendency of Beam, caused by Reaction and Weight ..... 75 

.101. Forces Acting upon One-half of Beam .......... 75 

102. Calculating Section Modulus : Beam Centrally Loaded ....... 75 

103. How Centrally Loaded Beam Tends to Bend 76 

104 to 106. Calculating Section Modulus of Beam ......... 76 

107. Strut under Load 78 

108. Diagram Indicating Stress in Part of Strut ......... 78 

109. Beam, under Vertical Shear, assumed to Consist of Separate Blocks .... 79 

110. Beam, under Horizontal Shear, assumed to Consist of Separate Planks .... 79 

111. Shearing Stress in Cantilever which Carries Concentrated Load at Outer End ... 80 

112. Shearing Stress in Cantilever which Carries Uniformly Distributed Load .... 80 

113. Shearing Stress in Cantilever which Carries Uniformly distributed Load and Two Concen- 
trated Loads 80 

114. Shearing Stress in Beam which Carries a Central Concentrated Load .... 81 

115. Shearing Stress in Beam which Carries Uniformly Distributed Load. .... 82 
116. Shearing Stress in Beam which Carries Uniformly Distributed Load and Three Concentrated 

Loads 82 

117 and 118. How a Single Reinforced Beam Resists Compression and Tension ... 86 
119. Diagram showing Proportionate Stresses above and below Neutral Axis to Produce Deforma- 
tion 87 

120. Finding Position of Neutral Axis . . 91 

121. Section through Cross Rods . . 99 

122. Section through Longitudinal Rods ... 99 

123 and 124. Double Reinforcement in Beams . . 100 

125. Designing Beam having Double Reinforcement . . 101 

126. Section of Tee Beam and Method of Finding Total Compression ... . 104 

127. Section at Centre of Span of Tee Beam fixed at Ends 110 



LIST OF ILLUSTRATIONS xiii 

FIG. PAGE 

128. Section, at Ends of Span, of Tee Beam fixed at Ends . . . . . . .111 

129. Determining Stress in Ordinary and in Reinforced Concrete Beams . . . . .113 

130. Vertical Shear in Reinforced Concrete Beam carrying Uniformly Distributed Load . .115 
131. Horizontal Shear in Reinforced Concrete Beam carrying Uniformly Distributed Load . 116 
132. Diagrams showing Need for Lateral Reinforcement in Columns ..... 117 

133 and 134. Rectangular and Circular Columns with respectively Rectilinear and Curvilinear 

Laterals . . . . . . . . . . . . . . .118 

135. Designing Column Eccentrically Loaded .......... 122 

136. Stress in Column Eccentrically Loaded .......... 122 

137. Influence of Angle of Repose and Line of Rupture and Design of Retaining Walls . . 123 

138. Three General Types of Retaining Walls. 124 

139. Retaining Wall with Cantilevers under Footpath ........ 125 

140. Retaining Wall to Resist Water Pressure . . . . . . . . .125 

141. Vertical Cross-section of Typical Factory Building in Reinforced Concrete . . 126 

142. Mixing Stage 128 

143 to 146. Four Patterns of Iron Tamper 128 

147. Perforated Spade . . .128 

148. Ross Spade 129 

149. Tamper for producing Fine Surface .......... 129 

150. Special Spade for Facing 129 

151. Andrews Tamper 129 

152. Wooden Tamper 129 

153. Cutting Anvil and Hammer 129 

154 and 155. Wrenches for Bending Ends of Bars ......... 130 

156 and 157. Wrenches for Bending Ends of Stirrups 130 

158. Key, or Twister, for Bending Ends of Stirrups, . . . . . . . .130 

159. Curry Tyer 130 

160. Elevation and Plan of Bench Bending Machine . . . . . . . . 130 

161. Kennedy Bar-bending Machine, No. 1 . . . . . . . . . 130 

162. Kennedy Bar-bending Machine, Geared Pattern . . . . . . . .130 

163. Making Bend to Given Inside Measurement ......... 131 

164. Making Bend to Given Outside Measurement . . . . . . . . .131 

165. Making a Double Set 131 

166. Making Sharp Bend in Thin Bar . . . . .131 

167. Photographic View of Right and Left Double Bar-bender ...... 131 

168. Method of Bending Small Bars 132 

169. Hennebique Square Pile ............ 133 

170. Coignet Round Pile 133 

171. Considere Octagonal Pile 133 

172. Lidgerwood Pile-driving Engine and Reinforced Concrete Pile ...... 134 

173. Simplex Pile . 136 

174. "Compressol" Borer, Rammer, and Tester 137 

175. " Compressol " Frame and Borer in Use . . . . . . ... . 138 

176." Compressol " Pile 138 

177. Plan of Typical Pile Caps and Connecting Beams ........ 139 

178. Section showing Pile Caps, Filling, Beam, and Slab 139 

179. Foundation Slab Reinforcement, held in Notched Templates ...... 140 

180. Ra^t Foundation Beam Reinforcement on Wooden Supports ...... 140 

181. Raft Reinforcement, Kingsway Church, London ........ 141 

182. Completed Raft, Kingsway Church, London ......... 142 

183. Plan of Retaining Wall, with Tapering Counterforts . . . . . . . 142 

184. Reinforcement of Retaining Wall, Royal Insurance Building ...... 143 

185 and 186. Reinforced Concrete Retaining Wall for Royal Insurance Building . . . 144 
187. Diagram showing the above Retaining Wall if built with Brickwork . . . . 144 

188. A further View of Retaining Wall, Royal Insurance Building, in Course of Erection . . 145 
189. Cross Sections of Columns showing Right and Wrong Methods of Placing Reinforcements . 146 
190 and 191. Horizontal and Vertical Sections through Typical Factory Staircase . . . 147 

192. Hollowed-out Concrete Block to Facilitate Spacing of Bars 147 

193. Money Order Office, Holloway, showing the Centering for Pillars, Floors, and Walls. . 148 
194. Money Order Office, Holloway, showing Two of the Wings in Course of Construction . 149 



xiv LIST OF ILLUSTRATIONS 

FIG. PAGE 

195. Method of Supporting Reinforcement in Beam Mould . . . . . . .150 

196. Typical Floor with Continuous Mesh Reinforcement. ....... 151 

197. Typical Floor with Sheet Mesh Reinforcement ........ 152 

198. Craig Screw-bore 154 

199. The Wainwright Steel Kerb .154 

200. Steps with Steel Kerbs 154 

201. Section through Column with Steel Corner Bars . . . . . . . . 154 

202. Ebco Corner Bar 154 

203. Setting Out Piers of Rectangular Building . .157 

204. Horse used in Setting Out 157 

205. Setting Out Acute Angle of Building 157 

206. Setting Out Obtuse Angle of Building 157 

207. Form for Square Pile 158 

208. Bolted Form for Bound Pile 158 

209. Stayed Form for Round Pile . 158 

210 and 211. Elevation and Enlarged Cross Section of Pile-making Platform .... 159 

212. Plan and Elevation of American Pile-making Forms and Platform . . . . .159 

213. Enlarged Cross Section through Form shown in Fig. 212 ...... 159 

214. Concrete Piles and Forms Dissociated .......... 159 

215. Foundation Form Built after Slab is Hard 159 

216. Forms for Pile Caps, Foundations, etc. . . . . . . . . . .160 

217. Typical Form for Foundation 160 

218. Cheaper Type of Form for Foundations . . . ... . . . . .161 

219. Form Strengthened with Wire 161 

220. Typical Column Form, with Slid-in Front Boards 162 

221. Typical Column Form, with Spiked-on Front Boards ....... 163 

222. Clamped Form for Short Columns 164 

223. Cheap Type of Column Form 165 

224. Column Form with Two Sides held between Fillets and Battens ..... 166 
225. Column Form with Two Sides held between Fillets and Battens ..... 167 

226. Typical Beam Form .168 

227. Boards with Splayed Edges to allow for Expansion . . . . . . 168 

228. Beam Form strutted from Extended Base Battens ........ 169 

229. Folding Wedges under Dead Shore 169 

230. Form for Two Intersecting Beams ........... 170 

231. Elevation and Cross Section of Forms for Pillar and Beam with Splayed Angles . . 171 
232. Two Cross Sections through Hennebique Floor and Plan of One Bay .... 172 

233. Beam Form held by Clamp .... 172 

234. Column Form with Bolts and Thumbscrews ......... 172 

235 and 236. Beam Forms used in Messrs. Sainsbury's Premises, London .... 172 

237. Beam and Column Forms used at a Bermondsey Warehouse . . . . . . 173 

238. Plan of Column Form 173 

239. Plan showing Position of Clamps for Reduced Column Form ...... 173 

240. Isometric Sketch of Forms for a Complete Bay of a Warehouse Floor .... 173 

241. Early Stage of Forms and Centerings at H.M.'s Stationery Office ..... 174 

242. Beam and Column Forms used at H.M.'e Stationery Office ...... 175 

243 to 245. Adjustable Beam Forms designed by H. Kempton Dyson . . . . .175 

246. Centering for Beams and Floors in Storied Building ....... 176 

247. Form for Twelve-sided Column 177 

248. Form for Fluted Column 177 

249. Form for Diminished Column ............ 177 

250 and 251. Columns at Wesleyan Hall, Westminster ........ 178 

252. Beam Forms, etc., at Wesleyan Hall, Westminster . . . . . . . . 179 

253. Form for Column Base at Wesleyan Hall, Westminster ....... 180 

254. Centering for Floor at Wesleyan Hall, Westminster ....... 180 

255. Centering Resting on Flanges of Steel Joists . . . . . . . . .181 

256. Centering Suspended from Flanges of Steel Joists . . . . . . . .181 

257. Centering for Concrete Floor having Steel Main Beams . . . . . . .181 

258. Centering for Arch Ceiling between Joist Flanges . . . . . . . .181 

259 and 260. Floor Centering supported by Hangers ........ 181 



LIST OF ILLUSTRATIONS xv 

FI - PAGE 

261. Form for Wall : Part raised to Second Position . . . . . . . . 182 

262. Section of Wall Form 182 

263. Panel for Wall Form .182 

264 and 265. Plan and Cross Section of Shored-up Form for Wall ... . 183 

266. Form for Simple Foundation Wall .......... 184 

267. Ransome's Wall Form . . . . . . . .' . 184 

268. Panel Shutter .184 

269. Collar and Set-screw at x (Fig. 267) . . 184 

270. Side of Form for Imitating Masonry Wall 185 

271. Form and Centering for Staircase ........ . 185 

272. Stringers and Carriage of Stairs ......... 185 

273. Form for Steps .185 

274. Metal Panel Form for Walls ........ 186 

275. Method of fastening Panel Flanges together . . . . . . . . . 186 

276. Detail of Metal Beam Form ............ 186 

277. Ransome Form for Cornice . ... . . . . . . . . . 186 

278. Form for Spandrel Wall to Bridge . . . . 186 

279. Form for Curtain Wall with Moulded Cornice ........ 186 

280. Two Sections of Form for Cornice to Hollow Coping ....... 187 

281. Form for Ornamental Parapet ........... 187 

282 and 283. Vertical and Horizontal Sections of Form for Battered Retaining Wall . . 187 

284 and 285. Plan and End Elevation of Form and Centering for Retaining Wall at Bridlington 188 

286. Part End Elevation and Section of Centering for Retaining Wall at Local Government Offices 189 

287. Diagram, Plan, and Details of Centering for Retaining Wall at Local Government Offices . 189 

288 and 289. Plan and Section of Centering, etc., for Retaining Wall in Deep Basement . . 190 

290 and 291. Elevation and Plan of Form and Centering for Silo . . . . . . 191 

292 and 293. Vertical Section and Plan of Centering for Dome of Wesleyan Hall, Westminster 192 

294. Centering for Dome at Annapolis New Academy 193 

295 to 297. Centering for Octagonal Dome, Circular in Section ...... 193 

298 and 299. Reinforcing the Large and Small Half Cupolas of the Poti Cathedral . . .194 

300 and 301. Centerings for Belfry and Arches of the Poti Cathedral 195 

302 to 305. Centerings and Reinforcement for Cupolas and Vaulting at the Poti Cathedral . 196 

306. Centering, etc., for Chimney at Northfleet 197 

307. Centering for Flat Bridge 198 

308. Centering for Melan Arched Bridge. . . . ... . . . . 198 

309. Elevation of Centering for Arched Principal, Hammersmith Baths ..... 199 

310. View of Centering for Arched Principal, Hammersmith Baths. .... 199 

311 and 312. Centering for Arch Ring Bridge 200 

313. Part Elevation and Detail of Centering for Bridge at Teufen ...... 200 

314. Detail of Bridge Centering (see A, Fig. 313) . . 201 

315. View of Part of False-work for Bridge at Teufen 201 

316 and 317. Centering for Almandares Bridge, Havana . . . . . . . . 201 

318. Centering for Meadow Street Bridge, Pittsburg 202 

319 and 320. Centering for Bridge of 233-ft. Span 202 

321. Centering for Bridge over River Werra in Thuringia . . . . . . . 203 

322. Centering for Bridge of 229-ft. 6-in. Span 203 

323. Centering for Small Bridge ...''. . . .203 

324. Centering for Walnut Lane Bridge, Philadelphia . 204 

325. Centering for Bridge of 126-ft. Span 204 

326. Centering for Bridge of 138-ft. Span 204 

327. Centering for Bridge of 80-ft. Span . . 205 

328. Centering for Bridge of 110-ft. Span . . . .205 

329 and 330. Centering for Bridge at Deer Park, U.S.A. . 205 

331. Centering for Flat Bridge of 42-ft. 7-in. Span 206 

332. Centering for Flat Bridge of 88-ft. 6-in. Span . . 206 

333. One of the Three Spans of the Chickahominy River Bridge 206 

334. Elevation of Panel of Suspended Centering ......... 206 

335. Section of Panel of Suspended Centering. ......... 206 

336. Cables supporting Concrete Sections, showing Key Spaces to be filled in . . . . 207 

337. Arch Ribs before Striking Suspended Centering . . 207 



xvi LIST OF ILLUSTRATIONS 

FIG. PAGE 

338. Arch Bibs formed in Suspended Centering ......... 208 

339. Centering for 6-ft. Sewer 208 

340. Centering for 8-ft. 6-in. Sewer 208 

341. Centering for Conduit at Jersey City 209 

342. Centering for 5-ft. Arched Culvert 209 

343. Centering for 8-ft. Arched Culvert 209 

344. Form for Small Box Culvert . . 209 

345. Collapsible Steel Centering for Sewer 210 

346. Conduit at Woolwich, showing Collapsible Steel Centering in Use ..... 210 

347 and 348. Form for Square Tank 211 

349 to 352. Form for Circular Tank 211 

353 and 354. Form and Centering for Gasholder Tank 212 

355. Form for Gasholder Tank at San Sebastian 213 

356. Gasholder Tank at San Sebastian 213 

357. Elevation, Plan, and End View of Form for Tapered Square Posts ... .214 

358. Cross Section (enlarged) through Form for Tapered Square Posts . . . . .214 
359. Plan and Elevation of Multiple Form for Tapered Square Posts ... . 214 

360. Section of Form for Triangular Posts 214 

361 and 362. Armoured Tubular Floor 215 

363. Coignet Beam and Slab 216 

364. Coignet Beam Reinforcement Consisting of Group of Small Bars . . . . .216 
365. Beam Supporting Floor Slab Centering .......... 216 

366. Section through Coignet Column ' t . 216 

367 and 368. Base of Coignet Column 216 

369 and 370. Coignet Pipe or Conduit 216 

371. Corr Bar Beam " Unit " .217 

372. Types of Corr Bars 217 

373. Dentile Floor with Mitre Tiles .218 

374. Dentile Floor with Bridge Tiles .218 

375. Dentile Floor with " L " Tiles .218 

376. Diamond Mesh Expanded Metal .218 

377. Bib Mesh Expanded Metal .218 

378. Section of Expanded Steel Bar . 219 

379. Four Types of Expanded Metal Floors . . 219 

380. Hennebique Stirrup round Tension Bar . 220 

381. Hennebique Tension Bars and Stirrups .... . 220 

382. Simple Hennebique Beam 220 

383 and 384. Hennebique Beams Continuous over Intermittent Supports .... 220 

385. Beam Beinforcement with Compression Bar and Double Stirrups ..... 220 

386. Hennebique Column 220 

387 and 388. Base of Hennebique Column ....... . 220 

389. Sheet Piles .220 

390. Hollow Diaphragm Pile . .220 

391. Square Section Indented Bar 221 

392. Bound Section Indented Bar . .221 

393. Johnson's Steel Wire Lattice .... .222 

394 to 397. Four Methods of Supporting Mesh-reinforced Floors 222 

398. View of Kahn Bar 223 

399. Section and Elevation of Kahn Trussed Bar 223 

400 and 401. Lintel Beinforced with Kahn Bar ... 223 

402. Keedon Beam Beinforcement ... . 223 

403. Keedon Column Beinforcement . . 22& 

404. Column and^Beam Beinforcements, Keedon System 223 

405. Lock-woven Mesh ... . 224 

406. Lock- woven Mesh Floor . . . 224 

407. Fireproof Construction with Lock- woven Mesh. . 224 

408. Floor Slab Supported by Four Columns, Mushroom System 224 

409. Head of Column, Mushroom System . . 224 

410. Paragon Stirrups . .225 

411. Paragon Column, Beam, and Floor Slab . 225 



LIST OF ILLUSTRATIONS xvii 

FIG. PAGE 

412. Paragon Column Hoopings ............ 25 

413 and 414. Paragon Helical Column Wrappings ......... 225 

415. Piketty Beam .............. 226 

416 and 417. Piketty Beams with Two and Three Tension Bars ...... 226 

418 and 419. Cross Sections of Piketty Beam with Four Rows of Bars ..... 226 

420 and 421. Square Piketty Column ........... 226 

422 and 423. Round Piketty Column 226 

424. The Upper Storeys of a Reinforced Concrete Warehouse at Cologne, Front View . . 230 

425. End View of Reinforced Concrete Warehouse at Cologne 230 

426 to 428. Vertical and Horizontal Sections of Small Hall with Arched Ribs . . . 231 
429. Interior View of Hall with Arched Ribs . . . . . . . . .231 

430 to 432. Vertical and Horizontal Sections of Small Hall with Barrel Roof . . . 232 

433. Interior View of Hall with Barrel Roof .......... 232 

434 to 436. Vertical and Horizontal Sections of Small Hall with Modified Barrel Roof . . 233 

437 to 439. Vertical and Horizontal Sections of Small Hall with Pierced Arched Ribs . . 234 

440. Interior View of Hall with Pierced Arched Roof ........ 234 

441. Interior View of Hall with Arched Ribs and Vertical Columns . . . . . 235 

442. Part Longitudinal Section of Hall with Arched Ribs and Vertical Columns . . . 235 

443. Cross Section of Hall with Arched Ribs and Vertical Columns ..... 236 

444. Part Longitudinal Section of Exhibition Hall with Three Flat Saucer Domes . . . 237 

445. Part Plan of Exhibition Hall . 237 

446. Interior View of Exhibition Hall 238 

447. Cross Section of Exhibition Hall through one of the Domes 238 

448. Part Plan of Ceiling of Exhibition Hall 238 

449 and 450. Elevation and Horizontal Section of Reinforced Concrete Fa9ade suitable for Club 239 

451 and 452. Detail of Reinforced Concrete Fa9ade in Elevation and Vertical Section . . 240 

453. Small Hall with Mask Walls of Brick and Principals and Roof of Reinforced Concrete . 241 
454 to 457. Longitudinal Section, Plan, Front Elevation and Cross Section of Hall with Reinforced 

Concrete Principals and Roof ........... 242 

458. T-piece for Sand-blast Apparatus ........... 244 

459. A Type of Bush Hammer . 244 

460 to 462. Front and End Elevations and Plan of " Granolithic Plate " .... 245 

463. Sand Concrete, 1 : 2. Full Size 246 

464. Crushed Stone Concrete (Cement 1, Yellow Bank Sand 2, and f-in. Screen Stone 3). Full Size 246 
465. Pebble Concrete with Scrubbed Surface (Cement 1, Bar Sand 2, and iVin. White Pebbles 3). 

Full Size 247 

466. Granite Grit Concrete (Cement 1, Bar Sand 2, and J-in. Granite Grit 3). Full Size . . 247 

467._Pebble Concrete (Cement 1, Bar Sand 2, and Screened Yellow Pebbles 3). Full Size . . 249 

468. Sand Concrete, 1:3 249 

469. Californian State Normal School at San Jose ......... 251 

470. Single-air-space Mortar Blocks (Sand and Portland Cement) Laid in Fireclay, after Firing 

and Quenching .............. 260 

471. Double-air-space Mortar Blocks, after Firing and Quenching 260 

472. Terra-cotta Three-air-space Partition Tiles Laid in Cement Mortar, after Firing and Quenching 260 

473. Various Building Stones Laid in Cement Mortar, after Firing and Quenching . . . 260 

474. Concrete Blocks Laid in Fireclay, after Firing and Quenching 261 

475. Granite Blocks Laid in Cement Mortar, after Firing and Quenching . . . .261 

476. Graph Showing Relative Water Absorption of Concrete 267 

477. Section through Floor Slab supported by Two Beams ....... 273 

478. Cross Section through Beam Centering .......... 274 

479. Horizontal Thrust at Crown of Arch 288 

480. Three Forces Acting on Half of Arch .......... 288 

481. Finding Line of Thrust in Arch 289 

482. Forces acting on Arch to Produce Equilibrium ........ 289 

483. Three-hinged Arch . . .290 

484. Two-hinged Arch . . 290 

485. Foot-bridges at Railway Terminals, Vera Cruz ...... . 290 

486. Foot-bridge at Railway Terminals, Vera Cruz .... ... 291 

487. Foot-bridge over Weisseritz, at Cotta-Dresden 292 

488. Moortown Bridge. Wimborne, Dorset 293 



xviii LIST OF ILLUSTRATIONS 

FIO. PAGE 

489. Decking and Parapets of Bridge in Cowley County, Kansas ...... 293 

490. Underneath View of Main Floor Span of Bridge in Cowley County, Kansas . . . 294 

491. Elevation and Details of Bridge in Cowley County, Kansas ...... 294 

492. Bridge over Merrimac River, New Hampshire ........ 295 

493. Foot-bridge at Mizen Head 296 

494. Erecting the Eibs for Foot-bridge at Mizen Head ........ 296 

495. Suspended Centering for Bridge ........... 297 

496. Bridge Crossing the Bremba, Italy . . 297 

497. Bridge at Kiel Dock Works 298 

498. Oceanside Bridge over San Luis Key River, California ....... 300 

499. Scenery Hill Viaduct, Philadelphia . 301 

500. Meadow-Street Bridge, Pittsburg 302 

501. Sectional Plan, Elevation and Details of Meadow Street Bridge, Pittsburg . . . 303 

502. Grafton Bridge, Auckland 304 

503. Bridge over River Werra, Thuringia . . . . . . . . . . 306 

504. Elevation and Plan of Bridge over River Werra ........ 307 

505. Details of Beams for Bridge over River Werra ........ 307 

506. Plan and Elevation of Walnut Lane Bridge, Philadelphia 308 

507. Bridge near Teufen, Switzerland 309 

508. Retaining Wall with Asphalt Damp-proof Course .311 

509. Timbering to Support Earth, before Building Retaining Wall (R. A. C.'s Building) . . 312 

510. Shuttering and Reinforcements for Retaining Wall (R. A. C.'s Building). . . . 312 

511. Retaining Wall Complete (R. A. C.'s Building) .313 

512. Inside Face of Retaining Wall, and Methods of Supporting Mains (R. A. C.'s Building) . 314 

513. Section Showing Front Vaults (R. A. C.'s Building) 315 

514. Plan of Grillage Foundation (R. A. C.'s Building) 315 

515. Section through Top Layer Beams of Grillage ........ 315 

516. Section through Bottom Layer Beams of Grillage ........ 315 

517. Trench for Grillages under Stanchions (R. A. C.'s Building) ...... 316 

518. Asphalt Damp-proof Course under Base of Back Retaining Wall (R. A. C.'s Building) . 317 

519. Section of Steel Pile 317 

520. Top of Finished Steel Piling 318 

521. Cast-iron Cap to Head of Steel Pile 318 

522. Reinforcement and Centering for Floor (R. A. C.'s Building) 319 

523. Elliptical Reinforced Concrete Lintels 320 

524. Shuttering to Curb of Skylight over Elliptical Vestibule (R. A. C.'s Building) . . .321 

525. Shuttering and Reinforcements for Sloping Roof (R. A. C.'s Building) .... 322 

526. Shuttering and Reinforcements for Sloping Roof (R. A. C.'s Building) .... 323 

527. Reinforcement around Elliptical Openings in Sloping Roof (R. A. C.'s Building) . . 324 

528. Shuttering for Swimming Bath 325 

529. Reinforcements for Swimming Bath (R. A. C.'s Building) 325 

530. Reinforcements for Swimmng Bath (R. A. C.'s Building) 326 

531. Reinforcing Sides of Bath and Concreting First Layer of Bars in the Bottom . . . 327 

532. System of Handling Concrete at Royal Automobile Club's Building .... 329 

533. Columns and Arched Ribs in the Royal Liver Building ....... 330 

534. Arched Beam (before Concreting) in Royal Liver Building . . . . . .331 

535." Skeleton " of Royal Liver Building . 331 

536. Cathedral at Manila, Philippine Islands 332 

537. Various Sections through Nave of Manila Cathedral ....... 333 

538. Longitudinal Section (looking north) and Details of Manila Cathedral .... 334 

539 and 540. West and East Elevations of Manila Cathedral 334 

541. Interior View of Manila Cathedral (Looking West) ........ 335 

542. Interior View of Manila Cathedral (Looking East) 335 

543. North Elevation of Manila Cathedral with Plan of Organ Room (A) and Section of Window 

Jambs (B) 336 

544. Details of Nave, Manila Cathedral 336 

545. Details of Chancel 336 

546. Longitudinal Section through Poti Cathedral 337 

547 and 548. Elevation and Horizontal Section of Dome of Poti Cathedral 338 

549. Ground Plan of Poti Cathedral . 338 



LIST OF ILLUSTRATIONS xix 

FIG. PAGE 

550 and 551. Cross Section and Plan of Wesleyan Hall ....... 339 

552. Key Plan showing Positions of Beams over Tea Room, Wesleyan Hall .... 340 

553. Section and Plan of Arch over Tea Room, Wesleyan Hall . . . . . . 340 

554. A Typical Pillar, Wesleyan Hall 340 

555 and 556. Details of Ceiling and Floor over Tea Room, Wesleyan Hall .... 341 
557 and 558. Details of Arched Ceiling to Basement Hall, Wesleyan Hall .... 342 
559. Cross Section, through Part of Arched Ceiling to Basement Hall, Wesleyan Hall . . . 343 

560. A Beam in the Floor of Conference Hall, Wesleyan Hall 343 

561 to 564. Sections through Floor of Conference Hall, Wesleyan Hall ..... 344 

565. Section through and Details of East Gallery, Wesleyan Hall ...... 345 

566. Free Church, Hampstead Garden Suburb, in Course of Construction .... 346 

567. South, West, and East Elevations of Free Church, Hampstead Garden Suburb . . 347 

568 and 569. South Elevation and Ground Plan of St. Barnabas' Church, Jamaica . . . 348 
570. Longitudinal Section through St. Barnabas' Church, Jamaica ...... 349 

571 and 572. East and West Elevations of St. Barnabas' Church, Jamaica . . . 349 

573. North Elevation of Chancel 349 

574. Details of the Reinforcement, St. Barnabas' Church, Jamaica ...... 350 

575. Sections through Nave and Chancel, St. Barnabas' Church, Jamaica . . . 351 

576. Balcony and Cantilevers in Majestic Theatre, Los Angeles ... . . . 351 

577. Cantilevers at Majestic Theatre, Los Angeles ......... 352 

578. Trussed Girder at Majestic Theatre, Los Angeles ........ 352 

579 and 580. Trussed Girder in Majestic Theatre, Los Angeles ...... 353 

581. Roof Plan and Details, National Gallery Extension. ....... 354 

582. Details of Intermediate and End Ribs of Dome Roof to National Gallery Extension . . 355 
583 and 584. Dome Roof to National Gallery Extension ....... 355 

585. Detail of Ribs to Dome, National Gallery Extension . . . . . . . 356 

586. Detail of Intermediate Ribs, Northern Gallery ........ 356 

587. Detail of Intermediate Ribs, West Gallery 356 

588. Detail of Pendentive . . .356 

589. Detail of End Ribs, Northern Gallery . . . 356 

590. Detail of End Ribs, West Gallery 356 

591. Bunker at Kinlochleven Aluminium Works ......... 357 

592. Section through Bunker at Kinlochleven Aluminium Works . . 357 

593. Detail of Beam of Bunker (A, Fig. 592) ... .357 

594 and 595. Plan and Section of Foundation Slab, Royal Insurance Building . . 358 

596. Details of Strong Room in Sub-basement, Royal Insurance Building . 359 

597 and 598. Vertical and Horizontal Sections of Dome over Main Entrance, Royal Insurance 

Building .360 

599 and 600. Walls with Post Buttresses ... .361 

601 to 604. Moulding Box for Fence Posts . . .362 

605. Fence Posts .362 

606. Reinforcement for Fence Post .... 363 

607. Iron Straining Post 363 

608 to 610. Reinforcement and Shuttering to Wall . . 364 

611. Retaining Wall, Dilworth Street, Pittsburg . .364 

612. Reinforcement of Heel of Retaining Wall 364 

613 to 616. Photographs, Elevation, Plan and Sections of Spiral Staircase . . 365 

617. Stairway in Mclntyre Building, Salt Lake City ... .367 

618. Staircases in Ritz-Carlton Hotel, New York . 
619. 247-ft. Chimney at Northfleet 

620. 144-ft. 9-in. Chimney near Drury Lane, London 369 

^70 
621. Sewer Pipe Reinforcements, Paris 

622. Sewer Construction at St. Louis, U.S.A. 

623 and 624. Sewer Construction at Acton, London 

625. Cross Section of Culvert at Kilton . 

626. End Elevation of Culvert at Kilton 

627. Cross Section of Conduit at Woolwich . 

628. Wharf at Dennemont .... 

629. Details of Wharf at Dennemont . 

630. Details of Jetty Head at Thames Haven .... ... 375 



LIST OF ILLUSTRATIONS 

FIO. PAGE 

631 and 632. Jetty Head at Thames Haven, in Course of Construction ..... 376 

633 and 634. Piles and Bracing of Thames Haven Jetty Head ...... 377 

635. Lifeboat Slipway, Ackergill ............ 378 

636 to 639. Lifeboat Slipway, Ackergill 379 

640 and 641. Columns and Arched Roofing of Kloof Nek Reservoir ..... 380 

642. Bridge forming part of Brooklands Motor Track ........ 381 

643. Stadium at Shepherd's Bush, London 382 

644. Platform Construction, Stadium 382 

645. Bradford Football Stand in Course of Construction ....... 383 

646. Floors, Columns, and Beams, Bradford Football Stand 383 

647. Details of Foundations and Columns, Bradford Football Stand ..... 384 

648. Plans of Bradford Football Stand 384 

649. Details of Beams and Brackets, Bradford Football Stand ...... 385 

650. Various Sections through Bradford Football Stand ........ 385 

651. Grand Stand, St. Paul, in Course of Construction ... . . . . 386 

652. Another View of Grand Stand, St. Paul, in Course of Construction ..... 387 

653. East and West Elevations of Grand Stand, St. Paul 388 

654. Half-elevation (to street) of Grand Stand, St. Paul . . . . % . . .388 

655. Railway Sleepers .............. 389 

656. Details of Sleepers 389 

657 and 658. Two Sections through Silos 390 

659. Section through Silo Walls 390 

660. Horizontal Section through Silos 390 

661. Horizontal Section through Silo Roof .......... 390 

662. Horizontal Section through Silos (looking upwards) ....... 390 

663. Horizontal Section through, showing Tunnel Arrangement ...... 390 

664 and 666. Water Tower at Singen 391 

666. Vertical Section of Water Tower 392 

667 and 668. Three Horizontal Half-sections of Water Tower . . 392 



Introduction; What Reinforced Concrete Is 



The Limitations of Concrete. For thou- 
sands of years concrete has been known 
and used as a building material possessing 
many valuable properties universality and 
consequent cheapness ; ease of handling, 
placing, and shaping ; ability to resist fire, 
water, and other destructive influences ; and 
great strength under compression. (A 
body, itself evenly and rigidly supported, is 
under compression when a load is placed on 
it tending to squeeze it.) Until the twenties 
of the nineteenth century, concrete was made 
by mixing together stones, gravel, etc., and 
lime, but such was the weakness of the 
material in tension (that is, when subjected 
to a force that tended to stretch it or bend 
it) that its employment had to be restricted 
to such applications as foundations, rela- 
tively thick walls, and the like. On the 
introduction of portland cement in 1824, 
the quality of concrete, particularly as 
regards the tensile strength, very greatly 
improved ; and modern cement manufacture 
a highly specialised industry has pro- 
duced a material with an appreciable ability 
to resist a stretching force. But good as 
the best cement is, it is still not good enough 
for use in a structure by itself except under 
compression. 

Reinforcement. Fortunately, it early- 
occurred to some ingenious workers that if 
they could use with it a second substance 
which would supply the tensional strength 
lacking in the concrete, they would then be 
in possession of a well-nigh ideal material 
for structural purposes. Tile, wood, bronze, 
iron, and steel have all been tried, and the 
preference goes to the last-named. Modern 
reinforced concrete, then, is simply a com- 
bination of a material (concrete) strong in 
compression but weak in tension with one 
(steel) that is itself strong in compression, 
but so much more strong relatively in 
tension. 

It may occur to some readers to ask why 
. steel should be used to bolster up a deficiency 



in another material. Why, it might be 
asked, is not the steel used alone, and full 
advantage taken of its valuable qualities ? 
Steel-frame construction was earlier in the 
commercial field than reinforced concrete, 
but it has not prevented the latter system 
from forging ahead and making a truly 
notable progress. There must be a good 
reason for that, and it is to be found in a 
number of considerations. Steel is relatively 
expensive ; the minimum quantity is used 
in reinforced concrete construction. A steel 
structure involves the making of thousands 
of riveted joints, and calls for a large amount 
of skilled work ; reinforced concrete con- 
struction is monolithic (literally, "as one 
stone "), and there is practically no jointing. 
Steel is corroded by atmospheric action 
unless immediately protected, and the pro- 
tection needs periodic renewal ; concrete is 
scarcely affected by atmosphere, requires 
no protection and no maintenance, and it 
preserves the steel even brighter than when 
it was inserted ; indeed, steel soon loses a 
coating of rust when embedded in concrete. 
Most important of all, an unprotected steel 
structure is the worst possible for resisting 
fire, by reason of the metal being such a 
good conductor of heat ; the steel rapidly 
expands as the temperature rises and con- 
tracts again as the cold water from the hose 
reaches it, thus pushing and pulling apart 
the elements of the structure and often 
causing complete ruin ; then its softness, 
should an extreme temperature be reached, 
robs it of its strength, and the steel girders 
become bent and twisted into a mere 
entanglement. On the other hand, concrete 
is a poor conductor of heat, and therefore well 
protects the embedded steel and localises 
any ill effects. Keinforced concrete possibly 
brings the goal of an absolutely fireproof 
structure within the attainment of our own 
time. 

Terminology. Keinforced concrete, in 
the commercial acceptance of the term, is 



REINFORCED CONCRETE 



concrete in which steel rods have been em- 
bedded to increase its strength. The term is 
somewhat vague, but no completely satis- 
factory substitute for it has been suggested. 
The material was (and often is) known on 
the Continent as " Monier concrete," but 
this does not explain itself, and may there- 
fore be dismissed. " Ferro-concrete " one 
of the best terms yet introduced has been 
monopolised as a trade name, although it is 
used as a generic term in technical literature ; 
even that designation, though, is not strictly 
correct, since " ferro " of course, is formed 
from " ferrum " or " fer " (respectively Latin 
and French for iron) ; but iron, as a struc- 
tural material, has long given place to 
steel, made by " alloying " iron with carbon. 
The term " reinforced concrete " does not, 
unfortunately, suggest the material employed 
with the concrete ; for example, at an early 
date concrete was reinforced with wood ; 
indeed, wood-reinforced concrete, known as 
" ligno-concrete," is now attracting some 
attention, but the term under discussion 
does not differentiate between " ferro- 
concrete," " ligno-concrete," or "sesso-con- 
crete " (bronze-concrete), the earliest type of 
reinforced concrete known. The term is 
open to another objection ; in a sense, the 
steel is itself just as much " reinforced " 
as is the concrete, and therefore " rein- 
forced steel " might seem as logical a de- 
signation. The term " concrete-steel " has 
its advocates, amongst whom we beg to 
take our place ; but it has not caught 
the imagination to the extent that some of 
the other terms have. Much the same 
can be said of " armoured concrete." It 
appears that the one term other than 
trade names likely to become universal is 
" reinforced concrete," and for that reason 
it has been adopted as the title of this book, 
the more precise " concrete-steel " finding 
a place in the sub-title. 

A Compound Material. Concrete-steel 
may be regarded as a compound material, 
and not as two distinct materials. The steel 
is wholly embedded in the concrete, and the 
adhesion between the two, after the maturing 
of the concrete, is such that a force sufficient 
to pull the bars from the concrete would re- 
quire to be 500 Ib. to 600 Ib. per square inch 
of surface contact, although it is regarded 
as safe to allow only 100 Ib. per square inch. 
Further, the two materials may be con- 
sidered as expanding and contracting at 
the same rates. The coefficient of expansion 



of concrete is -000006 (per 1 F. change of 
temperature), and that of steel averages 
0000066 ; the difference is therefore only 
1 in about a million and three-quarters. 
In spite of all this, there exists a division 
in the ranks of the concrete-steel engineers 
and specialists. On the one hand are those 
who believe that the natural adhesion of the 
concrete to the steel is sufficient to answer 
all ordinary requirements. On the other are 
those who say that experience teaches that 
this adhesion ought to be assisted by a 
mechanical bond, and they therefore " de- 
form " the bar to remove the least likelihood 
of its slipping when subjected to severe ten- 
sion, this being a time at which the sectional 
area of the bar is liable to be reduced, thus 
lessening or destroying the adhesion. De- 
formed bars are also used to allow of the 
employment of a steel having a greater 
tensile strength than mild steel. The chief 
Continental systems use plain bars, while 
the chief American systems use bars of 
special shape. This subject suggests other 
considerations, which would, however, lead 
us too far into the theory of the matter, 
and which will therefore be relegated to a 
later chapter. 

Comparisons. Stated in approximate 
terms, the compressive strengths of concrete 
and steel are as 1 : 28 ; the tensile strengths 
as 1 : 280 ; and the weights, bulk for bulk, 
1:4. To these ratios may be added those 
of cost, bulk for bulk ; let us assume the 
approximate costs of good concrete (mate- 
rials only) and steel to be 1 : 80, although 
this of necessity will vary. Working on 
these data, some interesting comparisons 
may be made. Figs. 1 to 3 show sections 
of circular columns, all assumed to be of 
the same height and capable theoretically of 
supporting the same weight. The concrete 
column must be 28 times the area in cross- 
section of that of the steel column, and 
nearly 1| times that of the concrete-steel 
one ; roughly, the relative areas would be 
about : steel, 4 ; plain concrete, 112 ; rein- 
forced concrete, 80. This comparison does 
not adequately show the true advantage of 
reinforced concrete, since the reinforcement 
in a tall concrete column is absolutely neces- 
sary, the rods and hooping preventing the 
concrete from bursting under a heavy load. 
Again, in the case of eccentric loading, one 
side of the column may be in tension, and 
steel is then required to take this stress. 
The relative sizes will depend upon the 



INTRODUCTION 



percentage of reinforcement, and the fore- 
going is based upon the minimum usually 
employed. In the case given, the area of 
steel in the steel column is twice that in the 
reinforced column, and it is this material 
that is the more expensive of the two. 
Again, the weight of the concrete column 



the relative costs (material only) working 
out as (roughly) 6-2, 1-7, and 1. This 
economy of steel is possible because all the 
steel in the reinforced concrete beam is 
placed as far as possible from the " neutral 
axis," and can therefore be stressed and 
utilised to its full value ; whereas in the 




AEEA-200 

Fig. 2 




AEEA- 5600 

Fig. 1 



S>[EEL- 100 <3. INO. 
TOJAL M?E.A- 4000 SQ. IH5 
Fig. 3 



Figs. 1 to 3. Columns of Plain Concrete, Steel, and Reinforced Concrete, respectively : 

Comparative Diagrams 



is 7 times that of the steel one, and about 
1*3 times that of the concrete-steel ; but 
the costs (for materials alone) will be approxi- 
mately in the proportions of 7, 20, and 
(nearly) 15. 

Now consider three beams, respectively of 
concrete, steel, and reinforced concrete, of 
the same depth and of the same value of 
resistance that is, capable of supporting 
the same load. Figs. 4, 5, and 6 are cross 
sections of these beams, d indicating depth 



steel beam, much of the metal is adjacent 
to the neutral axis, and part of its value is 
lost. (When a beam is loaded reasonably, 
it tends to bend, and its length is altered, 
but there is a layer of fibres or a plane, the 
length of which remains unaltered ; in a 
cross-section this " neutral surface " is, of 
course, a line, which is known as the " neutral 
axis.") 

It will be understood by the practical 
reader that the diagrams to this chapter are 





A 


X 


i 



PLAIN CONCRETE: 




Fig. 4 



ARtA= 3-3625 1 AietA or ajttL = '67^ i 

Fig. 6 



(Hi. 
Fig. 5 



Figs. 4 to 6. Beams of Plain Concrete, Steel, and Reinforced Concrete, respectively : 

Comparative Diagrams 



(the same in all three cases) and 6 breadth. 
Expressed in inches, the concrete beam has 
a breadth of 76 in., the steel beam -285 in., 
and the reinforced concrete beam 8 in. Their 
relative weights are (roughly) 70, 1, and 7. 
this might seem to show an advantage for 
the steel beam, but it will be noted that the 
steel beam contains nearly five times as 
much steel as the reinforced concrete beam, 



purely comparative, as steel is not commonly 
used in the form shown. If for the rect- 
angular steel beam were substituted a rolled 
steel joist having an equivalent moment of 
resistance, its area would exceed 3 sq. in., 
that is, more than five times that of the 
steel in the reinforced concrete beam. 

These comparisons show at a glance that 
it is cheaper to use concrete than steel to 



REINFORCED CONCRETE 



withstand compression ; but that to with- 
stand tensional stress concrete theoretically 
would cost two and a half times as much as 
the steel, but, practically, would be im- 
possible. Keinforced concrete effects the 
compromise between the two ; where it 



costs more than either steel or concrete 
alone, it offers advantages not possessed 
by the material with which it is compared. 
The parallel columns below put the case for 
the employment of reinforced concrete at 
a glance. 



CONCRETE 



STEEL (MILD) 



REINFORCED CONCRETE 



Weak in tension 
Strong in compression 
Cannot resist a strong shearing 

force 
A relatively cheap material 

Very heavy, strength for strength 

Fire-resisting 

Not attacked by weather and 
atmosphere ; low mainten- 
ance cost 

Easy to place and shape 



Very strong in tension 
Very strong in compression 
Can resist a fairly strong 

shearing force 
A relatively expensive material 

Very light, strength for strength 

Destroyed by fire 
Rapidly oxidised ; high main- 
tenance cost 

Difficult to place and shape 



Strong in tension 

Strong in compression 

Can be made to resist a strong 
shearing force 

Price competitive with that of 
any other system 

Lighter than plain concrete, 
strength for strength 

Fire-resisting 

Not attacked by weather and 
atmosphere ; low mainten- 
ance cost 

Easy to place and shape 



Historical Notes 



THE history of a practical science is seldom 
complete and proper without an early refer- 
ence to the ancient Eomans. The history 
of reinforced concrete is not one of the 
exceptions. Readers will be familiar with the 
oft-repeated statement that the Romans 
commonly used this system in the construc- 
tion of their public buildings ; but while this 
is an exaggeration of the truth, there are 
certain unassailable facts that stand out 
prominently. The Romans made good con- 
crete which can be seen to-day ; they antici- 
pated ligno-concrete (wood-reinforced con- 
crete) ; they combined tiles and concrete ; 
and they roofed the Frigidarium of the Baths 
of Caracalla with a coarse concrete, made 
with lime hydra ulicised with trass or volcanic 
scoria, and reinforced with bronze and iron 
rods. It is fairly certain, however, that 
there was no general knowledge at the time 
of the principle of reinforcement as it is 
understood to-day. There is no reinforce- 
ment, be it noted, in the Pont du Gard, a 
bridge in the south of France erected about 
56 B.C., and still in existence a worthy 
memorial to honesty of construction ; in this 
bridge the coarse material (stones) and 
cementing material (lime) were not mixed to- 
gether, but were apparently placed in alter- 
nate layers. It is difficult to believe that 
for eighteen centuries the idea applied by 
the old Roman bath builder could have lain 
dormant ; but while we know that concrete 
of a sort was commonly used, there is no 
evidence of the principle of reinforcing it, and 
so extending its usefulness, having been 
commonly applied at any time during that 
long period, unless exception is made in the 
case of the dome of St. Paul's Cathedral 
(1675-1710), in which, as is well known, Sir 
Christopher Wren caused chains to be 
embedded in concrete to help in resisting the 
lateral thrust. Certain of these chains were 
examined some years ago, and were found not 
to have corroded, although the concrete had 
been continually in a damp state. 

Early in the Nineteenth Century. 
It is not until 1830 that we find any definite 
mention of the idea of reinforced concrete, 
in which year was published J. C. London's 



" Encyclopaedia of Cottage, Farm, and 
Village Architecture," containing the sug- 
gestion that roofs might be constructed of 
cement in which were embedded iron tie- 
rods in the form of a lattice work, the whole 
being cased with flat tiles. Six years pre- 
viously (in 1824) Joseph Aspdin patented 
his method of making (portland) cement, 
and it had already (in 1828) been used 
extensively in the construction of the Thames 
Tunnel ; but for many years it had to fight 
opposition and was not in general use. For 
example, in the year 1840, when, as the 
result of a strike of carpenters, some fire- 
proof floors were constructed in Paris, they 
failed to become popular because a local 
material, gypsum (calcined to form what is 
now known as " plaster-of- Paris "), was used 
as the cementing material in the concrete, 
the result being that the embedded iron soon 
rusted. These floors were described in a 
paper read before the Royal Institute of 
British Architects in 1849 by G. R. Burnell, 
and more fully described five years later 
before that body by H. H. Burnell. Two 
systems were known ; in one the Vaux 
round rods, close together, were hooked at 
each end on to a flat wrought-iron bar lying 
on its edge ; in the other the Thuasne 
iron joists were employed, stirrups hanging 
from these and containing holes through 
which the round reinforcing rods passed. 
In each case the ironwork was embedded 
in plaster concrete. 

About the same time (July 9, 1840) a 
reinforced ceiling slab was patented by a 
Frenchman, Louis Leconte ; he proposed 
" the use of trusses of iron plates for floors, 
from which iron rods were suspended to carry 
a wire meshwork for sustaining plaster." 
There is evidence of an indefinite nature that 
shortly before this period English architects 
had constructed fireproof floors of concrete 
or brickwork in which flat iron bars were 
embedded. 

A Concrete Boat. A fertile period of 
invention, as relating to the subject under 
consideration, was the fifties of the nine- 
teenth century. Lambot, a French con- 
tractor, had proposed to build the hulls of 



REINFORCED CONCRETE 



boats with concrete, and at the first Paris 
International Exhibition (1855) he actually 
showed a flat-shaped boat, with sides 2 in. 
thick, made of hydraulic lime concrete 
reinforced with a skeleton of iron rods ; and 
in the same year he patented his invention 
in Great Britain. At the present time, 
boats are often built of reinforced concrete. 
Wilkinson. In spite of the systems that 
had been suggested, and of those that had 
already been practically tried, the chief 
credit for the invention of reinforced con- 
crete is commonly ascribed to William 
Boutland Wilkinson, who, on October 27, 
1854, patented a method of constructing a 
fire-resisting floor of concrete slabs rein- 
forced with a network of flat iron rods 
placed on edge or with secondhand wire 
ropes. Wilkinson was a Newcastle-on-Tyne 
plasterer, and as by 1852 there was estab- 
lished at Gateshead-on-Tyne, by William 
Aspdin, a cement factory, it may be fairly 
assumed that Wilkinson was thoroughly 
familiar with the use of cement concrete. 
We cannot do better than quote from the 
admirable " History " printed in the " Lock 
Woven Mesh Handbook." Wilkinson it was 
" who first suggested, in his 1854 patent, 
the use of a layer of sand kept wet upon the 
surface of a freshly made concrete floor, for 
the purpose of allowing the concrete to gain 
the maximum hardness. His patent also 
included the construction of hollow partition 
blocks very similar in detail to the inter- 
locking plaster partitions of to-day. The 
chief object, however, of the patent was the 
construction of fire-proof floors ; these 
Wilkinson proposed constructing both in arch 
form and flat, and he suggested reinforcing 
them with either flat iron bars placed on 
edge, or with secondhand wire rope. Wil- 
kinson states that the reinforcement was to 
be placed in the concrete to take the tension, 
and his drawings clearly show that he 
thoroughly understood the construction of 
such floors, for the reinforcement is placed 
in positions in the arch construction where 
it could give maximum service, while in 
floor slabs the rods or wire ropes were bent 
down at the centre of the span where the 
maximum bending moment occurs, but in 
continuous spans, or where built-in, were 
carried up to the top over the supports, so 
as to resist the reverse bending moment. 
The ends of the rope were also directed to 
be frayed out so as to bond in with the 
concrete at the ends of the span ; the advis- 



ability of this was evidently fully appre- 
ciated by Wilkinson, namely, to give anchor* 
age or mechanical bond and serve the same 
purpose as the fishtail or bend at the end 
of a rod in other systems of a later date. 
He also described and illustrated independent 
beams of reinforced concrete. His patent 
showed a grasp of most of the principles of 
modern reinforced concrete construction, and 
it is obvious that it was drawn up from 
thorough practical experience and perhaps 
some theoretical grasp of the subject." 

In course of time, Wilkinson erected a 
number of reinforced concrete buildings in 
Newcastle and elsewhere in the North of 
England, of which, however, no records are 
now known to exist. 

Francois Coignet. Less of the builder 
and more of the engineer was the French 
contractor, Francois Coignet, who in 1855 
patented, both in France and England, a 
system of forming floors by laying iron 
" planks," or rods, crossing each other, from 
wall to wall of a building ; iron beams could 
be placed to support the " planks " or rods. 
A false flooring is supported underneath the 
reinforcement and the concrete applied. 
Coignet had at an earlier date invented 
" Beton Coignet " (concrete for which the 
hydraulic lime and aggregates were mechan- 
ically mixed in certain proportions), and the 
advantage of reinforcement possibly occurred 
to him of itself, or he may have remembered 
the reinforced plaster-concrete experimented 
with some years before. In the years follow- 
ing the grant of his patent he carried out a 
variety of works, including a lighthouse at 
Port Said (Egypt), retaining walls in Paris, 
and thirty-three miles of aqueduct for the 
Paris water supply. He undoubtedly com- 
monly applied his system of reinforcement 
in the construction of these works, but very 
little is known as to the details. 

Dennett, Allen, Ransome, and Scott. 
In 1857 (patent dated March 9), a Notting- 
ham builder, C. C. Dennett, took advantage 
of the introduction of rolled iron joists and 
constructed a floor with reinforced concrete 
arches resting on _L-beams. The arch rein- 
forcement was of wood or iron. This floor 
was the prototype of the modern fireproof 
floor of steel joists and concrete. 

The 'sixties also saw a number of not- 
able introductions. In 1862 (January 30) 
Matthew Allen, a London builder, patented 
a system of building staircases, floors, etc., 
with which he afterwards did much business ; 



HISTORICAL NOTES 



iron bars 3 in. by J in., on edge, were 
embedded in concrete (cement 1, cinders, 
etc., 3) 2 ft. apart near the under-side of the 
slab, the bars crossing to form a network. 
The method was sufficiently practicable to 
be adopted in the construction of the 
Columbia Market, Shoreditch. Frederick 
Kansome, in 1865, suggested the construc- 
tion of girders, etc., by moulding a kind of 
cement around a hoop-iron skeleton. 

The year 1867 saw two important patents. 
One, by an Englishman, H. Y. D. Scott, 
related to a floor of concrete with inter- 
laced rods and hoop -iron or wire, supported, 
not on joists, but on wrought-iron tie-rods 
embedded in the concrete, and is of interest 
because the specification alludes to the use 
of " the tie-rods and hoop -iron taking the 
tensile strain, and the concrete the com- 
pressible," conclusive evidence that the 
basic principle of reinforced concrete was at 
that time understood. 

Joseph Monier. The other, a French 
patent dated 1867, by a Frenchman, Joseph 
Monier, was for the construction of plant tubs 
and the like by means of concrete reinforced 
with a meshwork of rods or wires. Monier 
started as a gardener, and developed into a 
manufacturer of gardening tools and appli- 
ances. He thought of substituting for the 
ordinary plant tub a vessel of concrete, but, 
finding this was brittle, he hit upon the idea 
of making a skeleton of iron network or 
trellis and then enveloping it with mortar 
or concrete. The second Paris International 
Exhibition (1867) contained examples of 
construction in the Monier and the Coignet 
styles. Monier followed up his 1867 patent 
with further inventions, patented in France 
in 1873, and, after the Antwerp Exhibition 
of 1879, at which he exhibited, he sold his 
inventions to G. A. Wayss (of the firm of 
Wayss and Co., Germany), who was soon 
instrumental in introducing the new system 
of building into a number of European 
countries. So closely was reinforced con- 
crete identified with Monier on the Continent 
outside France that it was commonly known 
as the " Monier system," a term replaced in 
course of time, and as other systems appeared, 
by the German -French " eisenbetonbau " 
and " betoneisenbau " (eisen [G.], iron ; 
beton [F.], concrete ; and bau [G.], build- 
ing or construction that is, " ferro-concrete 
construction "). 

A patent granted to T. Lythgoe and H. 
Thornton, in February, 1868, is of some 



interest. The inventors illustrate a floor 
consisting of _L-shaped iron bars suitably 
spaced and having a concrete filling between 
them ; a hoop-iron reinforcement laces the 
bars together, and passes alternately over 
and under them ; the bars and the reinforce- 
ment are wholly embedded in the concrete. 

It cannot help striking the student that 
the principle and almost the details of 
reinforcing were re-invented quite a number 
of times in the middle of the nineteenth 
century. Under the present laws British, 
at any rate it is doubtful whether any sys- 
tem already mentioned as being invented sub- 
sequent to 1840 when, as has been shown, 
reinforced floors were constructed in Paris 
could have been granted a valid patent. 
The Monier floor, introduced in 1873, is 
much the same as Allen's, patented in 1862 ; 
and Allen's was almost certainly anticipated 
by Wilkinson's, patented in 1854. 

Phillip Brannon. Brannon's provisional 
patent in 1870, completed the next year 
(October 12, 1871), shows that the inventor 
had grasped the principle of reinforcing con- 
crete. The specification of 1871 shows a 
concrete floor reinforced with an iron mesh- 
work, as well as the application of reinforced 
concrete to the construction of sea defences. 
Brannon was the first to suggest a reinforced 
concrete pile, the longitudinal reinforce- 
ments being of angle-irons united by bars 
riveted latticewise across them, the whole 
being wound spirally with wire. Brannon's 
patents were worked by a company formed 
by him and known as " The Monolithic 
Fireproof and Sanitary Construction Works, 
Ltd.," which in the ensuing years erected 
several reinforced concrete buildings, some 
at Walton-on-the-Naze, Essex, and two 
(houses) in Islington. 

Thaddeus Hyatt. Between the years 
1870 and 1877 much interest in the " new " 
system of construction was exhibited by 
Thaddeus Hyatt, an American who had 
become known from his invention of glass 
pavement lights. It is doubtful whether the 
published accounts of Monier's and Coignet's 
work in Europe had attracted his atten- 
tion. Between 1870 and 1877 he had a series 
of beams tested by Kirkaldy, and in these 
beams both single and double reinforcements 
were tried. He indulged in an interesting 
series of experiments, and these led him to 
believe in the necessity of anchoring the 
ends of the reinforcing rods, for which 'pur- 
pose he fitted them with nuts and washers 



REINFORCED CONCRETE 



to prevent their pulling through the con- 
crete. Between the years 1873 and 1881 
he obtained between thirty and forty differ- 
ent patents, most relating to reinforced con- 
crete ; and although he did much valuable 
work in assisting * engineers to a proper 
understanding of the theory of the rein- 
forced concrete beam, it does not appear 
that commercially Hyatt made any success 
with concrete construction. Hyatt's book 
showed that he regarded a reinforced con- 
crete beam as corresponding to a rolled steel 
joist, the steel rods being considered as 
equivalent to the bottom flange and the 
concrete as the top flange, the neutral axis 
being assumed to lie half-way up the beam. 
Later, a number of engineers made a similar 
mistake. 

From the 'Seventies to the 'Nineties. 
About the year 1870, a number of reinforced 
concrete sewer pipes were laid in Germany, 
and these are still in service. In 1877 an 
experiment was tried at Croydon by W. H. 
Lascelles, who applied a system based on 
patents obtained earlier, and built a number 
of cottages having a timber framework, the 
slabs being of concrete reinforced with 
diagonal rods. 

We now arrive at a period in which interest 
was being taken in reinforced concrete con- 
struction by engineers in all the progressive 
countries of Europe. Angelo Lanzoni, of 
the firm of Lanzoni, Galli and Co., made 
some applications of the system in Italy in 
1878, the first Italian patent being taken out 
by him five years later, anticipating Monier's 
Italian patent by four months. In Switzer- 
land, Monier's system was applied to the 
erection of small vaultings as far back as 
1880. In the same year, Rudolph Schuster 
came to terms with Monier, and in the suc- 
ceeding years carried out a series of works 
in Austria, these including large numbers 
of reservoirs and vats, vaulted flooring, fire- 
resisting doors, etc. Deep interest had been 
aroused in Great Britain, but the Monier 
system, for which a British patent was taken 
out, dated July 7, 1883, failed to become 
successful there. The activity was not con- 
fined to Europe, for in the United States 
where, in 1875, W. E. Wood had built a 
reinforced concrete house E. L. Ransome 
was building reinforced concrete warehouses 
about 1884, following later with a factory 
building, the Californian Academy of Science 
(architect, G. W. Percy), about 1888, and the 
Museum Building of Leland Stanford, Junior, 



University (architect, G. W. Percy), in 1892, 
the last-named building containing spans of 
45 ft. and being reinforced throughout. It 
is stated that the Academy building with- 
stood remarkably well the San Francisco 
earthquake of 1906. In 1884 Ernest L. 
Ransome patented in the United States a 
twisted square bar reinforcement, and in the 
preceding year John F. Golding obtained 
an American patent for " slashed metallic 
screening," otherwise expanded metal, which 
was employed as a lathing for plaster, 
its use as a reinforcement for concrete 
dating from about 1890. Benjamin Scarles 
took out American and British patents as 
far back as 1884 for the use of wire cloth as 
lathing, but even he had been anticipated. 

In the year 1885 W. H. Lindsay patented 
a system of reinforcement by which steel 
rods were passed through holes made in the 
webs of I- joists, one at the top and one at 
the bottom ; the vertical pairs crossed, and 
at the point of intersection they were made 
to form a loop through which was threaded 
a rod parallel with the iron joists. About 
the same time, too, a practical experiment 
was tried in Lincoln's Inn Fields, London, 
a block of offices with plain concrete walls 
and reinforced concrete floors being erected 
to the designs of William Simmons. 

From this point onward, we must omit the 
names of many patentees and inventors, 
as during the next twenty years or so there 
were hundreds of inventions relating to 
reinforced concrete, and scores of dif- 
ferent systems were exploited in Europe 
and the United States. 

In Hungary reinforced concrete was first 
applied in the erection of stables at the 
Artillery Barracks, Kassa, in 1886, by 
Robert Wiinsch, of Budapest. To take up 
the thrust of the vault, the separate metallic 
ribs were anchored, and the resulting system 
was at a later date widely applied in Austria 
and Hungary. Into Hungary the Monier 
system was introduced in 1887 by Wayss, 
being first adopted for a series of barrel- 
arched bridges. Merely mentioning by the 
way that on March 27, 1890, J. Mayoh 
patented in Great Britain the use of corru- 
gated plates on edge for strengthening con- 
crete floors ; that on July 7, 1891, C. A. 
Day patented in Great Britain the use of 
wire lattice suspended over steel joists for 
reinforcing concrete slabs ; and that in the 
same year Franz P. Meyenberg patented in 
the United States a floor constructed of 



HISTORICAL NOTES 



hollow terra-cotta tubes supported on steel 
rods embedded in concrete above and below, 
there being loose stirrups hooked round the 
bottom rods, we may pass to the period 
in which, in spite of all that had been done 
before it, reinforced concrete first compelled 
the serious attention of all progressive 
engineers. 

Edmond Coignet and Frangois Henne- 
bique. The two great names are those 
of Edmond Coignet (son of Francois Coignet) 
and Francois Hennebique, the former of 
whom, by applying the known principles 
of mechanics, evolved a system of calcula- 
tion that has proved remarkably truthful, 
and the latter of whom, basing his methods 
of calculation upon results obtained in prac- 
tice, has also made extremely important 
contributions to the technical consideration 
of the subject. Coignet as the scientific 
investigator, and Hennebique as com- 
mercial organiser, are properly regarded as 
" the pioneers of the modern evolution in 
the art of building." The story has often 
been told of the opposition which Coignet 
had to fight in getting the masonry of the 
proposed new system of main drainage in 
Paris in 1892 replaced by reinforced con- 
crete. He promised a large saving of money 
and of time required for construction, and 
his system, which was finally adopted, was 
carried out with complete success. Henne- 
bique, having organised a technical staff and 
licensed a large number of the most influ- 
ential contractors to work his system, was 
able to secure between the years 1892 and 
1899 work to the total value of two million 
sterling, representing three thousand con- 
structions, among the most remarkable of 
these being the bridge of Chatellerault, 
460 ft. long, comprising three arches, two 
of 133 ft. span and one of 167 ft. 

Hennebique's first patent dates from 1892 
(British patent, No. 14,530), and in this he 
demonstrates the utility of stirrups to rein- 
force beams against shear, in which matters 
he had to an extent been anticipated by 
Hyatt in 1877 and Meyenberg in 1891. In 
1897 Hennebique introduced cranked-up 
rods, and placed these one above the other, 
so as to reduce the width of the beam, 
following (to some extent) the lines laid 
down by Hyatt in 1877 and F. G. Edwards 
in 1892, in which latter year M. Koenen and 
G. A. Wayss, of Germany, patented in 
England a method of floor construction with 
rods cranked up at the point of contra- 



flexure, " the parts in tension being strength- 
ened by roughened or serrated metal rods 
or strips embedded in the structure." 

Progress since 1890. Speaking of the 
period following the years 1890 to 1892, 
N. de Tedesco, Ingenieur des Arts et 
Manufactures, and editor of Lz Citnent, has 
remarked that the modern method of build- 
ing remained for a long time in very few 
hands, and nothing was to be found in 
literature concerning the calculation of 
structures constituted by two different 
materials. Contractors were obliged to ex- 
periment in order to ascertain the most 
suitable proportions and arrangements of 
these materials for resisting the determinate 
load on a determinate span, but they were 
scarcely able to undertake with advantage 
investigations of that kind. Such work was 
rather the role of "ingenieurs des ponts et 
chaussees," but many of these attempted to 
co-ordinate the results of the tests carried 
out by Coignet and Hennebique, only to 
find that it was impossible to deduce mechan- 
ical laws from them, inasmuch as the 
mechanical properties of concrete are vari- 
able with the quality of sand or gravel, the 
cement used, the degree of fluidity of the 
mixture, the climatic influences during the 
hardening process, the age, and with many 
other surrounding conditions. Considere 
came to the rescue, and by a series of tests 
undertaken upon small specimens manu- 
factured under uniform conditions he was 
able to discover the laws governing the 
deformation of concrete submitted to stress. 
In 1890 Paul Neumann, in a published 
memoir, described his mode of calculation, 
which recognised the relation between the 
coefficients of elasticity of the two materials, 
steel and concrete ; and about the same 
time T. Melan published a treatise in which 
he came to practically the same conclusion 
as Neumann namely, that the elastic be- 
haviour of concrete was not the same under 
compression as it was under tension. Melan, 
by the way, invented in 1890 the system of 
constructing floors and vaulting named 
after him. The first important contribu- 
tion to a rational theory was the memoir 
written by Edmond Coignet and N. de 
Tedesco, and published in the Bulletin of the 
Societe des Ingenieurs Civils of France for 
March, 1894. Into Holland and Denmark, 
reinforced concrete was introduced about 
1890, the floors of the National Gallery 
at Copenhagen being built of reinforced 



IO 



REINFORCED CONCRETE 



concrete in 1891, the same system being 
used in the next year or so for har- 
bour works, and in 1894 for a 19-metre 
span bridge for foot passengers, just out- 
side Copenhagen. In Switzerland, where 
reinforced concrete had previously been used 
for vaulting, 1892 saw the introduction 
of Hennebique beams subjected to bending 
stress. Prof. F. Schule, of Zurich, has 
pointed out that two conditions favour 
the development of the new mode of con- 
struction in Switzerland : first, Switzerland 
has to import all its steel, and reinforced 
concrete involves a saving in this material 
not to be undervalued ; and, secondly, the 
excellent position occupied by Switzerland 
with regard to the manufacture of port- 
land cement. Both in Hungary and Italy, 
reinforced concrete was now making rapid 
progress, and by 1896 a bridge having a 
span of more than 25 metres was being 
erected at Sarajevo, in Bosnia. In 1897 the 
Hennebique system was introduced into 
Great Britain by (the late) L. G. Mouchel, 
Francois Hennebique's partner, and since 
then its progress in this country has been 
truly remarkable. 

Coming to the Paris International Exhibi- 
tion of 1900, the graceful palaces built by 
Edmond Coignet and Francois Hennebique 
attracted the favourable notice of architects 
and engineers the wide world over, and 
thousands of illustrated papers throughout 
the Continents published photographs of the 
Chateau d'Eau, erected by Coignet. If there 
was anything necessary to stimulate the 
adoption of reinforced concrete design by 
the engineers and architects of the day, it 
was afforded by this advertisement, from 
which dates an interest in the subject out 
of all proportion to that formerly existing. 

Of the few further inventions which it is 
worth specially to note may be mentioned 
the Siegwart beam, introduced in Switzer- 
land in the year 1902. The inventor, a 
Lucerne architect, desired to avoid con- 
struction on the building site other than 
the actual laying of the prepared beam, and 
his solution of the problem is satisfactory 
where the span does not exceed about 
5 metres. The Visintini system, invented 
in Austria and patented in Great Britain in 
1902, introduces framed beams into the 
reinforced concrete construction, and it came 
into practical employment in 1903, later 
coming into use for the vaulting of the 
Evangelical Church at Aussig. 



A specification of particular interest is 
No. 24,371 of 1904, the first British patent 
granted to Edmond Coignet. It describes 
and illustrates beams, floors, domes, roofs, 
columns, and walls, and has a high educa- 
tional value. The drawings of beams show 
the tension bars in the lower part of the beam 
to be connected to the compression bars in 
the upper part by means of ties. These ties 
also hook over the tension bars, which extend 
through the floor slabs and which are bent 
up to pass over the compression rods. 

Notable American Inventions. As a 
result of the rare commercial ability behind 
them, certain American systems have be- 
come extremely well known in Great Britain, 
and perhaps in consequence of this fact 
there is a common tendency to look upon the 
United States as the real " home " of 
reinforced concrete. While there is no 
doubt that the temperament of the Americans 
led them to welcome the " new " method 
of building (as, indeed, they welcome any 
promising invention), as a matter of fact, 
there is not, prior to 1892, much that is of 
truly American origin and of real note in 
the United States records with regard to 
reinforced concrete excepting the Thaddeus 
Hyatt inventions and the Ransome twisted 
bar, to both of which reference has already 
been made. In 1892, August C. Storck, of 
St. Louis, patented a method of strengthen- 
ing a concrete block with an embedded 
" brace " of slashed (expanded) metal ; and, 
in the following year, Thomas A. Lee, of 
Kansas City, filed a specification for a " non- 
shearing rod for strengthening concrete," 
two or more rods to be laid together spirally ; 
and E. L. Ransome took out what is appar- 
ently his first patent for a definite method of 
reinforcing concrete, his specification describ- 
ing and illustrating a slab supported at its 
ends by walls and reinforced by two bars, a 
shorter one above a longer one, the bars 
tapering to nothing at each end. 

Some years previous to this, Ransome had 
erected some important buildings in rein- 
forced concrete. 

In 1896, Edwin Thacher, of Detroit, filed 
an interesting specification showing a con- 
crete arch between abutments, the arch being 
reinforced with two series of flat metal bars 
in pairs, one bar of each pair above the 
other. At intervals, the bars had pegs which 
projected on each side, apparently to aid 
adhesion. Ira A. Shaler, of New York, 
patented in 1900 a method of reinforcing 



HISTORICAL NOTES 



ii 



slabs by means of a metal skeleton comprising 
longitudinal and cross members, the one 
welded to the other. Then came, in 1901, 
two important E. L. Eansome patents ; in 
the earlier, webs of hardened reinforced con- 
crete are placed in position, the spaces be- 
tween them spanned with falsework, and 
the top compression member of the floor 
then made with concrete ; in the later, a 
reinforced concrete floor extends to the 
exterior face of a building, and there becomes 
a belt course which caps the piers, a down- 
ward extension forming heads or lintels to 
the windows below. From this date the 
inventions become too numerous even to 



mention, but space must be found for a 
reference to the first Kahn patent, filed on 
December 11, 1902, and bearing date of 
August 18, 1903. It describes the now 
well-known Kahn bar, which has members 
projecting obliquely so as to form the 
diagonal members of a part of a truss. An 
improvement followed in a patent filed on 
May 4, 1903, and bearing date November 
3, 1903. These were the forerunners of 
a large number of patents by the same 
inventor, the assignees of the patents being 
the Trussed Concrete Steel Company, now 
well known both in the United States and 
in England. 




Concrete : Materials, Proportions 

and Mixing 



What Concrete is. Concrete is an arti- 
ficial stone made by cementing together 
fragments of hard material. The hard 
material, such as brick, stone, etc., is known 
as the " aggregate " (literally, something 
that has been gathered together) ; the 
cementing material, such as lime or cement, 
is termed the " matrix " (literally, but, in the 
case of concrete, not actually, a mould in 
which something is embedded). The active 
agents in the process of solidifying are the 
cementing material and the water ; the 
aggregate is inactive. 

Making a Strong Concrete. The 
strength and quality of concrete depend 
chiefly upon (1) the nature of the aggregate ; 
(2) the nature of the matrix ; (3) the pro- 
portions in which the materials (including 
water) are mixed together ; and (4) the 
efficiency of the mixing process. 

The compressive strength of concrete is 
limited by that of the aggregate, and the 
tensile strength cannot exceed that of the 
cement bond. The ideal concrete can be 
pictured as a composite block of stone, 
brick, etc., in which all the pieces fit together 
perfectly, being attached one to the other 
by a film of cement. Such concrete is not 
attainable in practice, the shapes of the 
fragments preventing such extremely close 
fitting, and thus causing the formation of 
pockets, which become filled with a mixture 
of cement and sand. Better concrete than 
is generally made would result from the use 
of graded aggregates ; first, a very coarse 
aggregate would be selected, then a much 
finer aggregate to occupy the larger of the 
voids, then a still finer one to occupy the 
lesser voids in the aggregate, and finally 
the cement. According to much of the 
present practice, there are but two grades 
in the aggregate, the coarse (stones, gravel, 
etc.) and the fine (sand). 

A little thought will show that the strength 
of concrete may be affected unfavourably 
by any one of a number of conditions : Any 
pait of the surface of the aggregate not 
covered with cement ; any grain of sand 



without a cement coat ; any coarse particle 
of cement which may in the future undergo 
chemical change ; any defective particle of 
cement that was incapable of doing its duty 
when brought into contact with moisture 
in course of mixing ; any very fine dust 
from the aggregate, inert and useless ; any 
crack or fissure in the broken stone ; any 
hollow place resulting from insufficient tamp- 
ing, etc. Any one of these will constitute 
a source of weakness. 

Concrete-making materials will now be 
considered in detail. In preparing the 
information here presented on aggregates, 
care has been taken to make it conform to 
a schedule issued with the " Interim Keport " 
of the Special Commission on Concrete 
Aggregates appointed by the British Fire 
Prevention Committee. 

NATURAL AGGREGATES 

Gravel. This usually consists of smooth, 
round pebbles with adherent sand, clay, 
and carbonaceous matter. The large stones 
need to be broken, and the sand to be sifted. 
Clayey gravel must be washed, as raw clay 
seriously affects the strength of the concrete 
and retards the setting of the cement. It is 
generally wise, as a matter of fact, to wagh 
all gravel. Soft or shaly should be avoided. 

1 cub. ft. of coarse gravel usually weighs 
about 97 lb., and clean shingle about 93 Ib. 

Shingle. This is a kind of gravel found 
on the seashore, its particles being of a 
rounded shape. It may contain a small but 
significant proportion of absorbed free salts, 
not readily removed by washing. If coarse 
material, free from sand, is required, it is 
best to specify, " clean washed and broken 
shingle." 

Sandstone. This consists of grains of 
sand, usually quartz, held together by a 
natural cement, the nature of which may 
vary in different examples. It is far from 
being the best of the stone aggregates, owing 
to its absorbent nature, and its weakness 
when wetted. It cannot withstand abrasion, 
and, as a rule, is of doubtful value as a 



CONCRETE: MATERIALS AND MIXING 



fire resistant. When it must be used, care 
should be taken that it is dense, uniform, 
and as homogeneous as possible in struc- 
ture. The Interim Report before alluded 
to states that the weights of sandstones, 
limestones, quartzites, and rocks of similar 
character should not be less than 130 Ib. 
per cubic foot, the crushing strength not less 
than about 3,080 Ib. per square inch, and the 
water absorption not more than 8 per cent, 
of the weight of the stone after a 24-hour 
immersion. The aggregate after preparation 
should be free from all dirt, decomposed rock, 
clay and organic material. 

Limestone. As in the case of sandstone, 
the quality varies. The oolites, when of 
fine grain, answer very well as a concrete 
aggregate, especially certain crystalline lime- 
stones of the magnesian or dolomite series. 
Limestone consists mainly of calcium car- 
bonate (with or without magnesium car- 
bonate), deposited originally from water, or, 
in any case, formed through the agency of 
water. It should have the qualifications 
mentioned in the " sandstone " paragraph 
as regards weight and strength. An average 
weight is about 168 Ib. per cubic foot, and 
the average strength under compression about 
4,700 Ib. per square inch. Limestones are 
dangerous in case of fire, becoming converted 
into lime. 

Limestone, sandstone, etc., are sometimes 
used in a crushed form unscreened so that 
the very fine pieces and the dust take the 
place of sand, which can therefore be omitted. 
There is, however, a strong objection to the 
use of this material. No flour-like dust, 
except that of cement, ought to be incor- 
porated into concrete. Stone dust is quite 
inert, and should it collect, as is likely, in 
the form of small lumps throughout the 
mass, it will constitute a source of weakness. 

Argillaceous or clayey limestones and the 
softer and more shaly limestones are un- 
desirable aggregates. 

Chalk. This is a variety of limestone, and 
varies in hardness, the upper strata being 
soft, while the middle and lower strata are 
much harder, and make better concrete than 
some other materials. Even the top stratum 
forms a fairly good concrete for walls where 
the height is inconsiderable. It has been used 
for two-storey cottages having suspended 
concrete floors with satisfactory results, being 
found to resist frost effectually and to 
possess a considerable crushing stress. The 
fine sandy portions should be eliminated by 



screening and coarse, sharp pit or river sand 
or fine brick sand or shingle substituted. In 
view of the large chalk area in the south of 
England, and where better materials are 
often not available, it might be used to 
advantage for walls of buildings where 
economy in construction is desirable. 

Quartzite. This in many respects re- 
sembles sandstone, but is harder and denser, 
resists abrasion better, is not so absorbent, 
and is therefore a better aggregate. It 
actually is a sandstone which has become 
hardened by natural forces and heat. 

Flint. This very hard but brittle stone 
consists of grey or black siliceous matter, and 
its probable source was a tiny sponge around 
which colloidal silica has collected and coagu- 
lated from the surrounding water, thus form- 
ing the peculiar shapes in which flint occurs. 
Flints are generally found in bands in one 
of the chalk series, but they also occur on 
the seashores, where they have been deposited 
after the erosion of the chalk beds. 

Flints obtained from the surface of 
arable land are somewhat dirty and re- 
quire washing, which is more easily per- 
formed after they have been broken or 
crushed. Flint cannot be broken by hand 
to make a suitable aggregate, but should be 
crushed by a stone breaker. 

Flint weighs about 162 Ib. per cubic 
foot, and the compressive strength is high, 
about 7,800 Ib. per square inch. Flint is 
liable to fly when subjected to heat, but the 
tendency is reduced by crushing to small 
particles less than f in. in size. It is not 
absorbent, and is not durable under shock. 
Shingle is largely water-worn flint. 

Granite, This is an igneous rock of 
greater strength under compression (exceed- 
ing 16,000 Ib. per sq. in.) and of varying 
specific gravity, an average specimen weigh- 
ing about 170 Ib. per cubic foot. 

Granite is an excellent natural aggregate 
providing that it is free from appreciable 
quantities of weathered or partially decom- 
posed material. The chief objection to its 
use is its high specific gravity, which tends 
to make it settle out of the concrete when 
the latter is being placed in position, but 
with a fairly stiff concrete the danger of 
settlement is small. The granite must be 
crushed to pieces of suitable size and sifted 
free from sandy and dust-like particles by 
screening over a J-in. mesh. The granite 
should be well watered immediately before 
use, as some varieties are highly porous and, 



REINFORCED CONCRETE 



if used dry, greatly reduce the strength of 
the concrete. If the watering is effected by 
means of a hose it will incidentally wash 
off any loosely adherent material, the pre- 
sence of which is detrimental to the concrete. 

The value of granite depends on the sharp- 
ness and angularity of the fragments as well 
as on the roughness of the surface. Those 
granites should, therefore, be chosen that 
are naturally brittle, and those which pro- 
duce rounded fragments in a crusher should 
be avoided as far as possible. The maximum 
size of granite particles is usually taken at 
| in., but for very large blocks fragments up 
to 2 \ in. in diameter are used. 

There is little difference in the quality of 
granites from various sources, so far as their 
value as aggregates is concerned, so that 
the chief attention should be paid to select- 
ing a granite of clean appearance, with as 
rough a surface as possible (to give a good 
key to the cement), and one of which the 
fragments are highly angular. Naturally, 
where a granite can be obtained locally, that 
will be the granite selected for a particular 
case. 

Other Volcanic Rocks. These include 
basalts, lavas, pumice, etc. They should be 
free from soft portions, and should be tested 
by immersion in water for 72 hours, at 
the end of which time there should be no 
signs of expansion, disintegration, or solu- 
tion. Basalts, traps, dense lavas, etc., 
should have a thoroughly vitrified structure, 
be homogeneous, and show a clean fracture. 
The basalts make really good aggregate, 
being strong (compressive strength, about 
19,000 Ib. per square inch) ; they are durable 
under abrasion, and they are of low water 
absorption ; but they have two disadvantages 
they are rather heavy (180 Ib. per cubic 
foot), and they have no marked resistance 
to fire unless care is taken to break them 
up small. Not so hard as the basalts, traps, 
etc., are the lavas, including pumice, which, 
however, for use as aggregates, should be 
moderately hard, show a bright, silky frac- 
ture and be free from dust and impurities. 

ARTIFICIAL AGGREGATES 

Burnt Clay. Under this sub -heading 
may be included broken bricks, clay ballast, 
broken terra-cotta, broken pottery, etc. 
The broken bricks should be such as, if 
whole, would be suitable for building pur- 
poses ; all soft and under-burned portions, 
should be discarded. Broken bricks should 



be free from old mortar, and from dust 
that will pass through a J-in. mesh. Most 
broken bricks are very absorbent, and for 
this reason should be watered before mixing 
with the other ingredients, as otherwise the 
cement is robbed of moisture and the setting 
is injuriously affected. Burnt clay ballast 
always needs to be inspected carefully for 
unburnt particles, and it is best to test a 
sample by soaking in water for a few days 
to see whether it disintegrates or not ; it 
needs watering in the same way as the 
broken brick. Porous terra-cotta and un- 
glazed earthenware make good aggregate, 
and these also should be watered before use ; 
the material should be clean, hard, and well 
burnt, but any large flat pieces or pieces 
of distinctly curved shape should be regarded 
with suspicion, since they do not work in well 
with the other pieces, and, by "bridging," 
may easily form cavities. 

Residues. These include blast-furnace 
slag, clinker, coke breeze, and cinders, which 
have been proved valuable for special appli- 
cations, being light and fire-resisting ; but 
it is of the utmost importance that sulphur, 
if present in anything more than a mere 
trace, be eliminated, as otherwise it tends to 
combine with the cement during the process 
of setting and seriously affects its strength. 
Then, too, any dense combustible material 
present (as coal) is a defect which militates 
against the successful use of residues. 

If materials containing sulphur are exposed 
to the atmosphere for a time and moved 
occasionally, the sulphur tends to disperse. 
The smell of the materials when moved is 
the simplest evidence of the presence of 
sulphur. 

Blast-furnace Slag. This should come 
from pig-iron smelting furnaces, and basic 
slag should be rejected. If there is any 
doubt as to the sulphur content, and in 
cases where the source of the slag is known, 
the works chemist should be asked for the 
analysis, which in many cases he should 
be able to supply without much trouble. 
Only a mere trace of sulphur is permissible, 
and the slag should be washed to remove 
dust and possibly some of the sulphur. 
Washing will also remove any free lime. 

Coke Breeze. The Interim Keport, of 
which mention has already been made, 
recommends that coke breeze for use as 
aggregate shall be entirely coke taken from 
gas retorts, coke ovens or special furnaces, 
and be absolutely free from clinker, coal, 



CONCRETE: MATERIALS AND MIXING 



and all substances that will not float in 
water, and from any admixture of material 
taken from the retort furnace or water pan 
below it, and from cinder, ash, or other 
adulterant. On no other aggregate is 
expert opinion divided to such an extent as 
on coke breeze. All agree that its light- 
ness makes it particularly adapted to sus- 
pended floors, and it has another advantage 
nails can be driven into it ; but it lacks 
strength. Its weight is about 35 Ib. per 
cubic foot. Being so very porous, it might 
be thought highly absorbent, but such is 
not the case, as many of the cavities formed 
by the bubbles of gas evolved when the 
coal is heated are sealed and therefore have 
no communication with the outside, and 
it is therefore only the surface (and 
broken) cavities into which the water can 
penetrate. Some coke breeze has so high 
a content of sulphur that its use is a posi- 
tive source of danger, but where this is not 
the case, the material is a valuable one. 
Much depends upon the kind of coal of 
which the breeze is the residue. Thomas 
Potter, in a paper read before the Concrete 
Institute, stated that he had used thousands 
of tons of coke breeze for floors, and did not 
remember a failure of any kind, and that he 
had proved it to be one of the best materials 
for floors and roofs. This, however, is not 
the experience of all users of the material, but 
this may be due to sifted ashes (" builders' 
breeze") having been substituted for true 
coke ashes. 

Clinker, Cinders, etc. Clinker used for 
concrete should be the thoroughly burnt 
and hard waste product of furnaces, free 
from dust, shale, or free lime. The quality 
of clinker, cinders, etc., varies greatly, but 
no material containing ashes, dust, rubbish, 
and more than a trace of sulphur is suit- 
able as an aggregate. In a later chapter 
(p. 261), the results of some fire tests of 
cinder concrete are given, and their some- 
what contradictory nature will be noted. 

Broken Concrete. Old and very hard 
concrete may be broken up so as to pass a 
H-in. ring, well screened in order to remove 
all dust, and then used as the aggregate in 
concrete for foundation work or rough 
walling ; but the material would not be 
sufficiently reliable for floors or fine work. 

On no account should old concrete in 
which calcium sulphate or plaster-of-paris 
was the matrix, as was the general practice 
before portland cement was used, be utilised 



as an aggregate, because when water is 
added it expands and ruptures the concrete, 
no matter how long since it was origin- 
ally employed. Failures of the kind have 
occurred where the plaster concrete was 
forty years old. 

SIZE OF AGGREGATE 

In no work ought the stones to be greater 
than 2J in. or 3 in. across, this size being 
used for large retaining walls, foundations, 
etc. For walls, the stones should pass a 
2-in. mesh and be retained on a 1-in. sieve ; 
and for floors they should pass a f-in. mesh 
and be retained on a ^-in. one. For the 
surface of concrete floors the stones should 
be broken so as to pass a f in. or \ in. mesh. 

The larger the stones, the stronger will 
be the concrete, the quantity of cement 
remaining constant. According to experi- 
ments undertaken by Messrs. Fuller and 
Thompson, concretes with stones 2 in., 1 in., 
and \ in. in diameter respectively required 
cement in the relative proportions of 6, 7, 
and 8 to be of the same strengths. The 
experiments proved, too, that round, water- 
worn material, such as gravel, gave a denser 
mass than broken stone, but that the latter 
gave the stronger concrete. The size of the 
aggregate particles greatly affects the pro- 
portioning of the ingredients. 

WASHING COARSE AGGREGATE 

It is nearly always desirable to wash the 
aggregate, and the work should be done on 
a wooden platform, inclined somewhat to 
allow water to run off freely. In preparing 
fairly large quantities for use, two men 
should shovel the material backwards and 
forwards, while a third keeps a constant 
supply of water playing on the aggregate, 
this being continued till the water runs off 
clear. The use of a mechanical washer 
some form of tower or tumbling drum gives 
better results. For good concrete the im- 
portance of washing the aggregate cannot 
be over-estimated. The water leaves the 
material in the proper condition for causing 
the cement to adhere closely to it, and the 
dampness undoubtedly assists the setting. 

SAND 

There appears to be a difference of opinion 
as to the function of sand in concrete. 
Some regard it as a cheapener to be added 
to the cement to increase its bulk and form 
a mortar in which the coarse aggregate is 



i6 



REINFORCED CONCRETE 



embedded. The writer does not share that 
view. The sand is essentially the fine part 
of the aggregate, and the proportion of it 
to be used depends not on the amount of 
cement thought to be necessary, but on the 
percentage of voids in the coarser aggregate. 
Then, in turn, the proportion of cement 
will depend on the percentage of voids in 
the sand and between the sand and the 
coarser aggregate, there being in addition 
enough to coat the whole of the aggregate. 
But for the trouble, sand of two sizes could 
be used, the finer occupying the voids in 
the larger. It is significant that experi- 
ments carried out by Messrs. Fuller and 
Thompson seem to prove that the strength 
and density of concrete are affected by 
variation in the size of sand grains, and 
that an excess of fine or medium sand 
decreases both strength and density. 

Sand must be sharp and gritty, and 
washed thoroughly free from everything 
that does not consist of small particles of 
stone, quartz, or non-plastic material. Dust, 
clay, earth, vegetable matter, and all other 
plastic substances, are sources of weakness, 
and the necessity of eliminating them invests 
with the greatest importance the washing 
and screening of pit sand. Kiver sand may 
not need washing, but is improved by screen- 
ing. Sea sand should have the salt washed 
out. Pit sand, when of glacial origin, is 
unsuitable ; so is sand containing more than 
10 per cent, of very fine grains. 

1 cubic foot of pit sand weighs about 
102 Ib. ; river sand, 106 Ib. ; coarse gravel, 
97 Ib. ; and clean shingle, 93 Ib. 

Substitutes for Natural Sand. Where 
sand is difficult to obtain, a substitute may 
be used. Indeed, the crushed stone sub- 
stitutes, washed perfectly free from dust, 
give a stronger concrete than do some sands, 
because the artificial crushing, when properly 
carried out, produces very sharp and angular 
particles. In addition to crushed stone, 
sand substitutes include ground burnt clay 
(pottery, bricks, etc.), slag sand (made by 
running molten slag into water whereby it 
becomes granulated), etc. All these need to 
be washed or screened to remove flour-like 
dust, because any material that passes 
through a sieve of 50 meshes to the linear 
inch is too fine to be satisfactorily used. 

Washing Sand. The best way to wash 
sand is in a stream of water, the rate of 
flow of which is adjusted to remove the par- 
ticles of clav, silt and rock-flour without 



carrying of! the sand. Some mechanical 
arrangement for keeping the sand in motion 
and to prevent the formation of eddies is 
necessary, or some portions will not be well 
washed. One of the most effective sand- 
washers consists of a large inclined pipe 
about 15 ft. in length, in which revolves a 
screw conveyor. A hopper is fitted to the 
lower end of the pipe, and through this the 
dirty sand is supplied, together with the 
necessary water. The water flows out at 
the upper end of the pipe carrying the clay, 
silt and rock-flour with it, and leaving the 
washed sand behind. As soon as the water 
flows away in a clear stream the water supply 
is cut off and the sand removed. This 
arrangement has been made continuous in 
action, and greatly improved by the addition 
of a second pipe about half-way along the 
larger one ; this second pipe carries off the 
washed sand, and delivers it in a very wet 
state into wagons or on to a receiving bed. 
Such an arrangement has been proved to use 
much less water in proportion to sand than 
the ordinary cylindrical washers generally 
employed. 

The use of sieves for washing sand is con- 
venient for quantities of not more than 
3 to 5 cwt., but is too slow and costly for 
larger amounts. The sand to be washed is 
laid on the sieve so as to cover to a thickness 
of not more than an inch, and the sieve is 
then partly immersed in a tub of water and 
shaken to and fro. The idea is that the 
finer particles will pass through the sieve 
and fall into the water beneath, but an 
examination of sand treated in this manner 
will show that it contains a considerable 
quantity of fine mud, which is too adhesive 
and remains suspended too readily in water 
to be removed by this treatment. Another 
method of sieve-washing consists in choosing 
a sieve with a rather coarser mesh than 
corresponds with the sand, and then washing 
the sand through the sieve into a tub of 
water. The coarse material remains on the 
sieve, and it is supposed that the clay and 
silt will be removed when the water is poured 
slowly out of the tub. An examination of 
the sand left will show, however, that it 
contains a considerable proportion of adhe- 
rent clay and rock-flour, and is not really 
satisfactory for use in concrete. 

Where no mechanical means of washing 
the sand is available, a tank about 6 ft. by 
6 ft. by 18 in. should be half-filled with sand 
and a stream of water turned in. The sand 



CONCRETE: MATERIALS AND MIXING 



is stirred continuously with a wooden pole, 
the stream of water being continued and 
allowed to overflow until it runs off perfectly 
clear. The remaining water is then run off, 
and the sand in the washer removed. 

Circular wash-mills, such as those used in 
the manufacture of cement, are not satis- 
factory for sand, the best sand washer being, 
as already mentioned, a stream of water 
flowing directly forward at a carefully regu- 
lated rate, in combination with some 
mechanical means to keep the sand in 
vigorous movement whilst it is immersed in 
the water. 

PORTLAND CEMENT 

From the times of the Romans until 
Smeaton's experiments in 1750 there was 
no material addition to our knowledge of 
mortars and cements. This intrepid experi- 
menter, urged by the necessity of finding a 
reliable mortar for use in the construction 
of the celebrated Eddystone lighthouse, 
upset all traditions by formulating the 
theory that the hydraulic or setting property 
of a lime-mortar did not depend on the 
hardness or whiteness of the stone from which 
the lime was made, but on the presence of a 
quantity of clayey or argillaceous matter 
interstratified with the stone. Aspdin in 
1824 patented portland cement, which was 
so-called from a fancied resemblance to the 
well-known Portland building stone. Chiefly 
owing to a lack of scientific knowledge 
amongst the earlier makers of portland 
cement, the first years of its history were full 
of failures, which tended to bring discredit 
on the industry and all concerned with it. 

Principles of Cement Manufacture. 
An endeavour will now be made to trace 
briefly the various stages in the experiments 
that led to the discovery of the principles 
underlying the manufacture of portland 
cement. The practice of burning limestone 
for the production of lime to be used as a 
binding agent or mortar is of great antiquity. 
The properties that distinguish the burnt 
lime from its original form of linestone are 
well known. Briefly, what occurs is that 
the calcium carbonate (CaC0 8 ) in limestone, 
when brought to a sufficiently high temper- 
ature, loses carbon dioxide (CO.,), and true 
lime (CaO) remains. Perfectly pure lime 
when treated with water forms calcium 
hydrate (Ca(OH) 2 ) or slaked lime, much heat 
is evolved, and the lime falls to powder. 
As a rule, the purer the lime the more 
2 



energetic is the reaction, and conversely 
the more impure it is the less intense does 
the action become and the less is the inclina- 
tion to fall to powder evinced. The chief of 
the impurities in a poor or slow slaking lime 
are sand or silica (SiO.,) and alumina (Al.,0 3 ) ; 
when combined these two substances form 
the basis of all clays or claylike materials. 

Smeaton first pointed out that the pres- 
ence of these clayey materials renders lime 
hydraulic (gives it the property of setting). 
James Parker was the first to use naturally 
occurring nodules of clayey matter which 
were sufficiently rich in lime to render them 
hydraulic when burnt and ground, and 
Aspdin was the first to use fairly pure lime- 
stone artificially mixed with clay. 

The table shows (roughly) the relative 
proportions of the principal constituents 
that are present in (a) a pure or " fat " lime, 
(b) a lean, partly hydraulic lime, (c) a natural 
cement, and (d) a modern portland cement :- 



Silica 
Alumina . 
Lime 



(a) 

1 
5 
98 




After burning, these constituents are no 
longer merely mixed together, but are 
chemically combined. That is to say, the 
lime does not exist as such, but with the 
silica and alumina it forms compounds 
known as silicates and aluminates and 
aluminosilicates, just as lime when slaked 
with water forms a hydrate ; thus, without 
chemical analysis, it is impossible to obtain 
as such either the lime or the silica and 
alumina. 

Portland cement may, therefore, be re- 
garded as containing compounds of lime 
with silica or alumina or both, its precise 
composition depending on many factors and 
varying slightly with each sample tested. 

Composition of Portland Cement. 
In order to follow more clearly the various 
steps in the manufacture of cement, it is 
desirable that the principles underlying the 
various processes should be understood. 
Portland cement is a combination of three 
bodies, and in order to bring about such a 
combination the interacting substances must 
be in such a condition that absolutely 
intimate union between the various particles 
may take place. When the reacting sub- 
stances are liquids, or solids that are soluble 
in a convenient medium, it is a compara- 
tively easy matter to bring about the desired 



18 



REINFORCED CONCRETE 



union (and this also is the case when union 
by fusion is employed) ; but although there 
is no reason why cement should not be made 
by mixing substances in solution (a method 
much used in the production of dental 
cements), it has been customary hitherto 
to heat the mixed materials to a state of 
incipient fusion or vitrification in the com- 
mercial manufacture of portland cement. 

Hence, to produce a satisfactory portland 
cement, the raw material must be brought 
to the finest possible state ; also, for physical 
and chemical reasons, the finished article 
must consist mainly of flour or inconceivably 
minute particles. Both the initial and final 
operations in portland cement making, there- 
fore, entail the employment of methods and 
machinery that are calculated to bring about 
a state of intimate union of the materials 
and uniform fineness of the product. 

The intermediate stage, in which the mix- 
ture is subjected to a temperature sufficiently 
high or prolonged to bring about the desired 
combinations, is where the greatest altera- 
tions and improvements of recent years have 
been effected. In earlier days the kilns, 
after being loaded by hand, required a week 
or more of firing, and then needed time to 
cool ; now, the mechanically fed and fired 
rotary kiln, electrically driven and con- 
trolled, turns out in a day more cement 
clinker than its almost obsolete forerunners 
produced in a week. For the successful 
production of a uniform and high- class 
cement, raw materials of as regular and 
unvarying a composition as possible are 
essential. 

Cement Works in England. Owing 
to the readily available deposits of chalk 
and clay in the Thames and Medway valleys, 
numerous works were started and still con- 
tinue in those neighbourhoods, and it was 
thought at one time that a good cement 
could not be produced save from these 
materials ; but whilst London cement still 
holds its own, the growth of knowledge and 
extended experience has shown that in 
many parts of Great and Greater Britain 
other materials exist from which some of 
the best cement may be manufactured, par- 
ticularly in the Tyne Valley, in Warwick- 
shire and Cambridgeshire. Probably, as 
landowners avail themselves of the know- 
ledge and experience now obtainable, an 
increasingly large number of factories will 
spring up in the neighbourhocd of the large 
centres of population where suitable raw 



materials and fuel may be found, together 
with a steady demand for a sound cement. 

Preparing Materials for the Kilns. 
Two chief methods are employed in pre- 
paring the materials for the kilns. They 
are known as the " wet " and " dry " pro- 
cesses respectively. The former is used 
chiefly for chalk, clay, and soft materials, 
and the latter for argillaceous limestones and 
harder rocks. In some works, a modifica- 
tion or combination of both methods is 
employed. 

The Wet Process. This is used for most 
of the cement made in England at the pre- 
sent time, and is chiefly applied to the vast 
deposits of chalk found in Kent and Essex, 
in which districts the chalk is obtained by 
quarrying, and the men employed in this 
work are known as " chalkies." The material 
is sometimes loosened by blasting, but this 
is a practice not often necessary. After 
removal of the bull-head or gravelly soil, 
the chalk is loosened by crowbar or pick 
and slides down the face of the cliff, and is 
directed by temporary wooden shoots into 
the wagons awaiting its descent. Large 
quantities of flints occur in layers in the 
chalk deposits, and these are removed either 
at the quarry or when the wagons are un- 
loaded at the washmill. The chalk wagons 
are conveyed to their destination by small 
locomotives. These locomotives also bring 
the clay in trucks from the pit, or, more 
generally, from the wharf whither it has been 
brought by barges from the dredging grounds 
on the Medway. In one of the most up-to- 
date works in Great Britain the clay is now 
dumped into the washmills at the pit. The 
object of the washmill is to separate the 
coarse impurities and to mix the fine ones 
together intimately. 

The washmill consists essentially of one or 
more circular tanks, usually about 15 ft. in 
diameter and 5 ft. deep. In the centre is a 
brickwork pier carrying a vertical shaft bear- 
ing radial arms from which is suspended a 
series of harrows. The central shaft is 
rotated by suitable gearing, and the steel 
teeth of the harrows run in the annular 
space between the central pier and the mill 
walls. Into this mill the chalk and clay are 
introduced in the proper proportions, to- 
gether with a sufficient amount of water to 
form with the disintegrated chalk and clay 
a liquid of creamy consistency known as 
slurry. Often the chalk and clay are treated 
separately in washmills, and the mixed 



CONCRETE: MATERIALS AND MIXING 



slurries are then treated in another mill to 
secure intimate contact. As the slurry 
leaves the mill it passes through screens and 
fine sieves, so that any coarse particles are 
retained in the mill. From the washmills 
the slurry is led through the grinding mills, 
which formerly consisted of French burr 
stones, but these are being gradually replaced 
by tube mills and more effective grinding 
machines. 

Both the materials led into the mills 
and the resulting slurry should be under the 
immediate control of the works chemist or 
his assistants. The rule-of-thumb methods 
of former times are now superseded by very 
careful sampling and speedy analysis which 
permit the composition of the mixture to 
be accurately adjusted, if necessary. As a 
general rule, the amount of chalk is about 
three times that of clay ; in some cases the 
mater als are weighed, and in other cases 
measured. In either case, the chemist 
should be able readily to alter the relative 
proportions of the ingredients, and this is 
generally accomplished by varying the 
amount of chalk, the clay remaining constant 
in weight or volume. As the slurry passes 
through the mill samples are taken at short, 
regular periods ; these samples are carefully 
mixed, and an average sample is then sent 
to the laboratory for analysis. This analysis 
consists generally of a speedy and reasonably 
accurate estimation of the calcium carbonate 
present, together with a test of the fineness 
-and moisture in order to ascertain whether 
the grinding apparatus is working satisfac- 
torily. 

Formerly the slurry was pumped from the 
washmills into huge settling tanks or backs, 
where it was allowed to dry, and it was then 
dug out and loaded into upright bottle 
kilns. After the slurry had been pumped 
into the backs there was little possibility of 
altering its composition, and so, right or 
wrong, it went into the kilns. At a later 
date, the slurry was pumped from the wash- 
mills into chambers or tunnels through or 
over which passed the waste heat from the 
kilns, and this was a decided economy, 
saving both time and labour. But this 
practice is now giving way to the method of 
burning in rotary kilns with previous mix- 
ing in tanks, the slurry being constantly 
mechanically stirred in order to obtain a 
material of more uniform composition. 
This method is also of great advantage to 
the chemist, as it affords another opportunity 



of testing the composition of the mixture 
and correcting it if necessary. 

The Dry Process. As already men- 
tioned, the dry process is often used for lime- 
stones, shales, etc., in the manufacture of 
portland cement, which cannot be reduced 
to powder or slurry merely by stirring them 
with water in a washmill. The chief 
materials for which it is used are the lime- 
stones and shales of the lias formations in 
Warwickshire, and the limestones and harder 
clays of Wales. The materials are dried 
and reduced to a fine powder by means of 
crushers similar to those used for grinding 
clinker. The powder is mixed with water 
into a stiff paste in a pugmill or mixer, con- 
sisting of an open trough fitted with revolv- 
ing blades on a horizontal shaft. The paste 
is broken into irregular lumps or is com- 
pressed into bricks, which are dried and then 
taken to the kilns. If a tube mill is used for 
crushing the material, an open pan mixer 
may be used. Where a rotary kiln is em- 
ployed, there is no need to dry the lumps or 
bricks of " compo," and some firms prefer 
to use a very soft paste and to load this 
direct into the rotary kiln. 

Opinions are divided as to the relative 
values of the cement produced by the wet 
and dry processes respectively. The choice 
of one or other method depends chiefly on 
the nature of the raw materials, so that a 
detailed comparison is of little value, and 
concerns the materials rather than the 
method employed. For this reason such 
comparisons are best avoided as being often 
misleading and based on insufficient grounds. 
It does, however, appear to be a fact that 
the clinker produced from materials treated 
by the dry method is somewhat denser and 
more difficult to grind than that obtained 
when the wet process is used. 

Producing the Cement Clinker. The 
mixed materials, in the form of liquid slurry 
or of solid lumps or bricks, must be heated 
sufficiently to cause it to fuse partially or to 
vitrify, in order to effect the necessary 
chemical combinations and to produce 
cement clinker. Until recently, this heating 
was effected in kilns of two types (a) vertical 
shaft or bottle kilns, and (b) continuous 
Hoffmann kilns. The former consist of an 
upright shaft into which fuel and lumps of 
cement mixture are fed so as to form alter- 
nating layers, in a manner resembling the 
burning of lime. When the kiln has been 
filled the fuel is lighted, and when it has all 



20 



REINFORCED CONCRETE 



been burned the clinker is withdrawn from 
the bottom of the shaft. This intermittent 
working produces a cement of variable 
quality, but, with care, satisfactory results 
are obtained. The Hoffmann kiln consists 
of a ring-shaped space in which the lumps 
of cement mixture are stacked, and the fuel 
is burned in hollow pillars, about 6 ft. apart, 
left in filling the kiln. Its action is quite 
continuous, one part of the kiln being filled 
and another emptied whilst others are being 
burned. Owing to the regenerative use of 
the air and fuel gases, this kiln is highly 
economical in fuel, but the labour of stack- 
ing the lumps of material and of withdrawing 
the clinker by hand makes it more expensive 
than the rotary kiln, which is rapidly re- 
placing all other types, one of its advantages 
being that it can burn liquid slurry. 

Although rotary kilns were introduced at 
the beginning of the eighties of the nine- 
teenth century, their extensive and success- 
ful use is a matter of comparatively recent 
history. The rotary kiln as now generally 
used may be briefly described as a steel or 
iron cylinder from 5 ft. to 9 ft. in diameter 
and from 75 ft. to 250 ft. in length ; many 
kilns originally of the former length have 
been extended, and now 150 ft. is the more 
general length. This cylinder is lined with 
a material that is calculated to withstand 
both the high temperature and also the 
action of the highly basic, almost molten, 
clinker. The cylinder is erected at an in- 
clination varying with the materials that are 
to be used and the intended speed of rotation, 
the modern tendency being to decrease the 
angle and increase the speed. The kiln is 
supported on roller bearings and set in 
brickwork hoods at each end, several kilns 
being often connected to one chimney shaft 
by means of flues. It is not necessary to 
enter into the details of driving and control- 
ling the machines ; electric power is usually 
employed, and by means of an ingenious 
device the speed of rotation and the fuel or 
raw material feed may be varied at will. 
Very finely powdered coal, which has been 
passed through a rotary drier before grind- 
ing, is the usual fuel ; the coal is brought 
from the grinding mills by screw or other 
conveyers to hoppers at the lower end of 
the kilns, and is blown into the latter by 
steam or air supplied by a fan. The raw 
mixture, or slurry, is pumped into the upper 
end of the kilns, down which it is carried by 
the rotary motion. During its passage 



through the kiln the moisture is first driven 
off, then the carbon dioxide is removed, 
and finally the lime thus produced combines- 
in the hottest part of the kiln with the silica, 
and alumina of the clay ; the semi-molten 
globules that are thus formed drop out at 
the lower end of the kiln, and then pas 
through cooling cylinders. Cold air passes- 
up through these cylinders, which in con- 
struction are very similar to the kilns ; and 
in this way the clinker is cooled and the now 
hot air is supplied to the kilns. The clinker 
thus produced is of a dark greenish colour, in 
pieces about the size of hazel nuts, and is- 
received into hopper trucks, which, as soon 
as they are filled, are conveyed to the weigh- 
ing bridge and thence on to the storage- 
hopper or dry grinding mill. 

Grinding the Clinker. Dry grinding 
with burr stones has now become quite 
obsolete, the work being done largely by ball 
and tube mills, pendulum mills, or other 
modern contrivances, no preliminary crush- 
ing of the rotary clinker being necessary. 

The form of pendulum mill chiefly used 
in England is the Griffin mill, consisting 
essentially of a horizontal driving pulley, 
from which the shaft is suspended by a 
universal bearing. To the lower end of 
the shaft a crushing roll is attached, which 
is thus free to swing in any direction within 
the pan containing the ring against which 
the roll works, the pulverising being done 
between the roll and the edge of the pan. 
Just above the roll is a fan, and on the 
under side are a number of ploughs. The 
roll is within the ring, and centrifugal 
action carries it against the edge of the 
pan. When the clinker is fed into the 
mill it is thrown up by the ploughs between 
the roll and the edge of the pan. When 
ground sufficiently fine the cement passes- 
out through a screen surrounding the pan, 
which for that purpose has a number of 
openings downwards thrdugh it. The whole 
grinding part of the mill is covered with a 
conical sheet-iron case, and the revolving 
fan draws in air through the top and forces 
the cement out through the screens. 

The ball mill, briefly described, consists- 
of a cylinder revolving round a horizontal 
axle. The circumference is formed of per- 
forated steel plates arranged to overlap each 
other. Outside the plates are coarse screens, 
beyond which again finer screens may be 
placed. Sometimes an air-separator replaces 
the screens. The clinker is fed into the mill 



CONCRETE: MATERIALS AND MIXING 



21 



through a hollow trunnion, and in the mill 
are placed a number of steel balls which are 
carried round and dropped from plate to 
plate as the mill revolves, thus grinding the 
material that is fed into the mill. 

As a rule, the ball mill is only used for 
coarsely grinding the clinker, which is then 
passed into a tube mill, which consists of an 
iron shell from 16 ft. to 22 ft. long and 
about 5 ft. in diameter. This shell is about 
half-filled with flint pebbles, and the mill 
is fed and discharged by means of hollow 
.shafts at each end ; as the mill rotates the 
pebbles are carried partly round up the side 
of the mill until a point is reached where they 
drop back in a cascade upon the material 
below, thereby reducing it, in time, to finest 
possible powder. The material to be ground 
enters at one end of the tube and gradually 
works its way to the other end, where it is 
discharged on to screens or into an air- 
separator, the coarse material being returned 
to the mill to be re-ground. The pebbles 
are fed into the mill through a manhole, and 
are prevented from leaving by the smallness 
of the apertures in the mill. The tube mill 
is only used as a finishing mill, and usually 
it must be worked in conjunction with a 
coarse grinding mill, a ball mill being 
admirable for this purpose. 

From the mills the now finely ground 
cement is carried by belt, screw, or other 
conveyers to the storehouse, and there 
packed in sacks or casks or stored in bins 
until required. 

Regulating the Setting of Cement. 
ement made from clinker burnt in shaft or 
Hoffmann kilns is relatively slow in setting, 
but that made under the best modern con- 
ditions and burnt in rotary kilns sets with 
inconvenient rapidity and requires the addi- 
tion of some retarding agent. Several sub- 
stances are available, the one most gener- 
ally employed being gypsum (plaster-of- 
paris) or other forms of calcium sulphate 
(CaS0 4 ). Most specifications of portland 
cement recognise the necessity for this 
retardation of the setting, and they permit 
the addition of not more than 2 per cent, 
of one of the substances above mentioned, 
the addition of the substance being made 
during the operation of grinding. An- 
other plan now frequently followed is the 
blowing of steam into the tube mill ; in this 
way any traces of free lime are hydrated. 
The cement thus manufactured should be fit 
for use immediately after grinding, which 



was not the case in former days ; it used 
to be inadvisable to use cement freshly 
made. On no account should the con- 
tractor keep cement long on hand before 
using. 

Testing Cement. In the early days of 
the cement industry the methods of testing 
employed were on all fours with those 
obtaining in the majority of the cement 
works of that period, namely, if existent at 
all, entirely unscientific, rough, and crude. 
The apparent antipathy of the earlier cement 
makers towards chemists or other trained 
scientific workers and methods was only 
overcome when, as a result of numerous 
disasters, their aid was perforce invoked. 
This probably accounts for the many useless 
tests introduced at a later period of the 
industry, every engineer, architect, or sur- 
veyor, in their anxiety to avoid disaster and 
disappointment, devising tests that pleased 
their individual fancy. At this time, also, 
every works of note had its own system of 
mechanical and physical tests, according to 
the ideas of those in control. This diversity 
of opinion and specifications continued until 
the opening years of the present century, 
when, in 1903, a Committee on Cement was 
appointed by the Engineering Standards 
Committee, which is supported by the prin- 
cipal engineering institutions and societies. 

The sectional committee on cement which 
drafted and revised what is now the recog- 
nised British Standard Specification (obtain- 
able through any bookseller for 5s.), is com- 
posed of representatives of various institu- 
tions, public bodies, cement manufacturers, 
experts, and large users. Every buyer or 
large user of cement is recommended to 
obtain a copy of this specification, and to use 
only cement that is guaranteed to comply 
with its requirements. It will at once be 
recognised that it is impossible for any in- 
experienced person to carry out many of the 
physical and mechanical tests therein de- 
scribed, whilst the question of ascertaining 
the chemical composition is one for the 
trained analytical chemist. The users of 
cement, therefore, must of necessity depend 
largely upon the guarantee of the manu- 
facturer or employ the service of an ex- 
perienced tester or expert. If, however, the 
quantity purchased, or the importance of the 
work, does not warrant the expense thus 
involved, the user, having taken the pre- 
liminary precaution of obtaining a well- 
known brand of cement, may for his own 



REINFORCED CONCRETE 



gratification carry out a few simple tests 
that will at once tell him whether the cement 
is suitable for the object in view. 

Before describing these " unofficial " tests, 
it may be well if we briefly enumerate the 
requirements of the British Standard Speci- 
fication. The following is abstracted, by 
permission of the Engineering Standards 
Committee, from Report No. 12 (revised 
August, 1910), " British Standard Specifica- 
tion for Portland Cement," which specifies 
the limits of the following properties : (a) 
fineness, (b) specific gravity, (c) chemical com- 
position, (d) tensile strength (neat cement), 
(e) tensile strength (cement and sand), (f) 
setting time, (g) soundness. 

For tests (d) and (e) special apparatus is 
required, whilst (b) and (c) can only be pro- 
perly performed in a chemical laboratory or 
specially fitted testing department. In order 
properly to determine (a), a good balance 
and set of weights and sieves are required, 
and for the official method of ascertaining the 
" setting time " (/) a Vicat needle is neces- 
sary. A little piece of apparatus must be 
obtained to carry out the Le Chatelier test 
for " soundness " (g). Each one of these tests 
requires some amount of experience before 
reliable and concordant results can be 
expected. 

It will at once be realised that cement 
testing is not work that can rightly be left 
to the " odd job " man, but requires care 
and a fair amount of intelligence and common 
sense. The tests formulated above are such 
as any normal, unadulterated, and well-made 
cement will, as a rule, satisfactorily pass. 

At the same time, it is possible that 
cements not absolutely in accordance with 
the requirements of this specification may 
prove satisfactory and reliable in practice, 
though the employment of cement that is 
guaranteed to pass the test enumerated is 
strongly advocated. 

Fineness Test for Cement. The object 
of determining the degree of fineness of 
cement is to ascertain that its particles are 
in such a condition as to practically con- 
stitute an impalpable powder. It may 
be definitely stated that it is only the 
" flour " in a cement that possesses any real 
cementitious properties. All the mcderately 
coarse particles are not only useless, but 
may prove to be a source of real danger. 
The determination of the fineness is a test 
that may be easily carried out by any 
intelligent man. In order to make the test 



quantitative, a balance and weights, and 
two " Standard Wire " sieves of 180 by 180, 
that is 32,400 meshes, and 76 by 76 (5,776) 
meshes, per square inch are required. One 
hundred grams (metric system) or 4 oz. of 
cement is carefully and continuously sifted 
for fifteen minutes on each sieve. The 
residue on the 180 sieve must not exceed 
18 per cent., that is 18 grams or '72 oz. 
(nearly f oz.), and 3 per cent, on the " 76." 
The latter constitutes practically an unweigh- 
able amount except a delicate balance and 
weights be used. 

Quantitatively, a rough test of the fineness 
of the cement may be made by sifting a 
handful of cement through the " 76 " sieve ; 
there should be practically nothing left on the 
sieve. 

Specific Gravity Test. This takes the 
place of the now obsolete " weight per 
bushel " test. It cannot be carried out 
except in a properly equipped testing-room. 
The object of this test is to eliminate 
adulterated or lightly burnt cement. 

The British Standard Specification requires 
the minimum specific gravity of portland 
cement to be 3-15 at works, and 3-10 after 
delivery ; taking the higher figure, 1 cub. ft. 
of solid cement would weigh 196 Ib. It is 
not easy to get uniform results in determin- 
ing the specific gravity of a fine powder, and 
the specific gravity will vary with the extent 
to which the cement is packed into the 
weighing vessel, and also with the conditions 
of manufacture. The accepted weight of 
1 cub. ft. is 90 Ib., although actually it may 
vary between 75 Ib. and 110 Ib. A cement 
ton varies slightly. It may be 2,200 Ib., 
made up of 10 two-cental sacks, or it may 
be 2,244 Ib., consisting of either 11 sacks, 
each of 204 Ib., or 12, each of 187 Ib. All 
these weights are net. 

Analytical Test. The services of an 
experienced analytical chemist should be 
employed if there is any doubt as to the 
genuineness of the cement. The addition 
of ground slag, bricks, or other adulterant 
can only be proved by accurate analysis. 
Certain definite ratios between the " basic " 
and " acid " components of the cement are 
stated in the Standard Specification, as well 
as limits for the percentages of magnesia, 
sulphuric anhydride, and water. 

Tensile Strength Test. The cement 
both " neat " and " with sand," is made up 
into briquettes under certain stated condi- 
tions, and the tensile stress of the briquettes 



CONCRETE: MATERIALS AND MIXING 



ascertained by pulling each piece until it 
breaks in a special machine after the bri- 
quettes have been stored for stated periods 
of time. A properly equipped testing-room 
and the services of an experienced gauger 
are required. 

"Setting Time" Test. The study of 
the " setting time " of a sample of cement 
when gauged with water is, although appar- 
ently simple, one of great complexity. In 
order to obtain concordant results it is 
necessary to strictly adhere to the conditions 
expressed in the Specification. A Vicat 
needle and gauging tools are necessary. The 
setting time may be roughly ascertained by 
gauging up a small portion of the cement 
with sufficient clean water to form a thick 
paste. A pat is then made up on a small 
sheet of glass or piece of slate, and the surface 
tested with the thumb-nail until it is im- 
possible to make any impression thereon. 
It is possible in this way to distinguish 
between a " quick " and " slow " setting 
cement, but not with that accuracy obtain- 
able with the standard needle. Care must 
be taken in gauging not to " work " the 
cement after it shows any signs of " setting." 

Soundness Tests. The object of all the 
suggested tests for " soundness " is to ascer- 
tain rapidly the probable effect of time upon 
the set cement. It is a practical endeavour 
to prove that the physical state and chemical 
composition of the cement are as nearly as 
possible correct. At present there are no 
fully satisfactory methods of attaining this 
object. The Le Chatelier test has been 
adopted by the Engineering Standards 
Committee because it is the only simple 
quantitative method that has yet been 
devised. Many other tests have been sug- 
gested, but they are either not quantitative 
or else require expensive and complicated 
or delicate apparatus. For a full description 
of the test, the British Standard Specifica- 
tion should be consulted. Briefly, it con- 
sists in filling a small mould, consisting of a 
split brass ring fitted with indicating needles, 
with the gauged cement. The mould and 
contents is then immersed in water for 
twenty-four hours, removed, and the distance 
between the indicating needles measured in 
millimetres. The apparatus is then placed 
in water again, which is caused to boil con- 
tinuously for six hours. When cool, the 
distance between the indicating points is 
again measured, and any increase noted. 
A figure is thus obtained which should not 



exceed the prescribed limits of the Specifica- 
tion. As before mentioned, this is the only 
simple quantitative test for " soundness " 
that can be performed without the aid of 
expensive and delicate apparatus. 

It may not be out of place, however, to 
briefly describe one or two quantitative tests 
that require practically no apparatus. Some 
people, before testing for setting time and 
soundness, spread out the cement to be 
examined in a layer 3 in. deep for twenty- 
four hours at a normal temperature; but 
the practice is not recommended. It 
should be understood that no reliable results 
can be expected unless the operator knows 
how to gauge cement properly and make it 
up into a pat on a piece of clean glass. It 
is not a simple matter to do this, and ex- 
perience is the only teacher. 

An excellent test is to plunge a pat, before 
the cement sets, into cold water. A good 
cement will easily stand this test, a faulty one 
will either fall to pieces almost immediately, 
or show signs of cracking at the end of 
twelve or twenty-four hours. A quantitative 
" hot " or accelerated test is to place a pat 
after setting into cold water, which is then 
gently caused to boil and kept boiling for 
six hours. The pat should remain on the 
glass, and not show any signs of cracking 
or disintegration. It is not serious if the pat 
leaves the glass, but is otherwise sound and 
not buckled or cracked. 

It may be remarked that the British 
Standard Specification does not require a 
cement to undergo or stand either of the 
tests last mentioned, but in most cases any 
cement that complies with the requirements 
of the Specification will also pass these 
qualitative tests. 

NATURAL CEMENTS 

" Natural " cement manufacture differs 
from that of portland cement, which is 
always regarded as an " artificial " cement, 
in one important particular : for natural 
cement, the material highly calcareous marl 
or septarian nodules is dug, calcined, and 
ground, thus producing a cement of varying 
composition and quality ; whereas, for port- 
land cement, additions are made to the raw 
material in accordance with the results of 
skilled tests, so as to obtain a cement of per- 
fectly uniform composition. The cement is 
also tested at various points in the manu- 
facturing process, and the resultant product 
is therefore a reliable and uniform article, 



REINFORCED CONCRETE 



whereas the natural cements vary greatly, 
even at the same works. 

A natural cement very inferior to 
portland cement is produced in Belgium, 
and a few years ago large quantities of it 
were used in Great Britain, the material 
being introduced into this country by the 
most unscrupulous methods. The packages 
were put up in close imitation of those con- 
taining genuine portland cement of British 
origin and often bore labels of well-known 
British brands. The importation of this 
fictitious portland cement is now practically 
dead, but as recently as 1904 it is computed 
that nearly 234,000 tons entered Great 
Britain. 

SLAG CEMENT 

Ordinary blast-furnace slag without any 
addition does not, as a rule, possess cementi- 
tious properties ; it requires the addition of 
lime powder to make it hydraulic. Cement 
made from blast-furnace slag, and known 
as " slag cement," has, therefore, chiefly 
consisted in the past of a mixture of granu- 
lated blast-furnace slag and finely powdered 
lime. In certain patented processes, a 
cement is made by causing the molten slag 
to become impregnated with alkaline salts 
in solution. 

A small proportion of granulated blast- 
furnace slag when finely ground with good 
cement clinker is stated to make the con- 
crete of which it forms part less pervious to 
water, and substantially to increase its 
mechanical strength ; burnt clay, trass, or 
other puzzolanic material, is said to have 
the same effect. There are, however, reasons 
for supposing that these statements have 
been originally made by those interested in 
the utilisation of slag cements or adulterated 
portland cements, and that they are subject 
to the suspicion of being biassed. 

Slag cements are now made by a variety 
of processes, and in some cases they resem- 
ble true portland cements so closely in com- 
position and properties as to be indistin- 
guishable from them. Most slag cements, 
on the contrary, are quite distinct from port- 
land cements, and are nothing more than 
artificial puzzolanas. Such cements should 
be rigidly excluded from admixture with 
portland cements or from sale under this 
title ; their nature should be clearly stated, 
as puzzolanic cements are very inferior to 
portland cements. Apart from the careful 
adjustment of the composition of the slags 



by the addition of limestone, followed by a 
recalcination of the mixture in the kiln 
(whereby a true portland cement is formed), it 
does not appear probable that there will be 
any improvement in the manufacture of 
cement from blast furnace slag that will 
render it a keen competitor of true portland 
cement. It must be remembered that slag 
is a waste product, and that in the manu- 
facture of iron from its various ores the com- 
position of the slag must vary according to 
the character of the ore smelted, without any 
consideration relative to its ultimate employ- 
ment as a cement. 

Iron Portland Cement. For some years 
a mixture of portland cement and granulated 
blast furnace slag has been used in Germany 
under the term " Iron Portland Cement," 
and has been the subject of much discussion. 
The portland cement is made by mixing the 
granulated slag with lime and heating to 
sintering, and the clinker so produced is 
mixed with about three-sevenths of its 
weight of slag. The mixture is then ground 
and forms the iron portland cement of com- 
merce. A commission appointed by the 
Prussian Minister of Public Works reported 
in 1908 that iron portland cement and port- 
land cement may be considered of equal 
value if in air-hardening tests under standard 
conditions the iron portland cement gives 
satisfactory results ; but this has not been 
borne out by the iron portland cements 
placed upon the British markets, these having 
a variable composition and often containing 
deleterious substances. 

LIME CONCRETE 

Concrete made with lime is weaker and 
more porous than that made with portland 
cement. Lime concrete will not always set 
properly in a damp soil, and it has therefore 
little value beyond that of a simple hard core. 
Chalk lime is quite unsuitable for damp 
situations, only the best hydraulic limes 
being permissible. The best lime that can 
be used for concrete is that known as blue 
lias lime, which is made from a limestone 
containing approximately 79 per cent, of 
carbojiate of lime and 17 per cent, of clay ; 
and it is the clay content that makes the 
lime hydraulic, that is, capable of setting 
under water. The less the proportion of 
uncombined lime present, the greater is the 
hydraulicity. Lime concrete is not used 
for reinforced work where portland cement 
is obtainable. 



CONCRETE: MATERIALS AND MIXING 



WATER FOR CONCRETE MIXING 

The water used in mixing the concrete 
should be clean and preferably fresh. The 
use of hard water causes a white efflorescence 
to come out on the walls after a time, similar 
to the well-known patches that appear on 
red-brick fronts. On no account should 
dirty water, or water containing organic 
matter, as that from stagnant pools, be used. 
Warm water causes the cement to set a little 
quicker. An excess or insufficiency of water 
in mixing is to be guarded against 20 gals, 
of water per cubic yard is a fair amount, i)ut 
a larger or smaller quantity may be necessary, 
according to the nature of the aggregate. 
Whether the concrete is wet enough can be 
judged after it is deposited and rammed ; 
there should be just a slight wetness of the 
top surface. When there is not enough 
water, the cement does not set properly ; 
where there is too much, the cement may 
be washed off the aggregate, and, if the 
forms are roughly made, some of the thin 
stuff will run away. Another point is that 
a moderately wet mixture prevents the form- 
ation of voids and secures sufficient plasticity 
to ensure a complete filling of the space round 
and below the steel reinforcement ; but an 
excessively wet concrete contains numerous 
globules of water, which, when absorbed, 
leave the concrete porous, and tend to accu- 
mulate on the surface of the reinforcement, 
particularly on the under side. According 
to the Proceedings of the American Society 
of Civil Engineers, the weakening of the 
bond from this cause was evident in certain 
beams in which the adhesion was noticeably 
weak, the water cavities being apparent at the 
bottom and sides of the steel bars. 

Sea-water should be avoided, as it con- 
tains many impurities. 

PROPORTIONING CONCRETE 

Correct practice in proportioning concrete 
is based upon a proper understanding of the 
purpose of the ingredients, which has 
already been explained. The cement, in 
addition to its general binding power, fills 
the interstices in the sand, and the sand the 
interstices in the aggregate. Thus, the 
coarser the aggregate, the more the sand 
required ; and the coarser the sand, the 
greater the proportion of the cement that 
must be used, though the addition of a small 
amount of a much finer sand is a more 
economical proceeding. The coarser the 
cement, the greater the porosity of the 



resultant concrete ; and by filling the fine 
interstices in the cement itself with a still 
finer substance (hydraulic lime, or extremely 
finely ground cement-flour), it is possible 
to produce a non-porous concrete. 

Round pebbles do not interlock with one 
another so well as do materials of an angular 
shape, and as a result more sand and cement 
are necessary to fill the spaces (in concrete 
parlance termed " voids "). 

The essential condition is the production 
of a solid body without voids, all the pieces 
of aggregate and particles of sand being 
united together by the cement matrix. 

Considered in the light of the above, 
concrete consists of but two component 
parts, aggregate and matrix ; and the sand 
may be regarded as the finer part of the 
aggregate, its purpose being merely to 
occupy the spaces between the coarser 
pieces. Thus, if the sand were omitted, 
cement to nearly the same bulk as that of 
the sand would have to be added. 

It follows, then, that although success in 
concrete making is often thought to depend 
entirely upon strict adherence to a formula, 
there is no one formula of any value for 
general adoption ; and the correct propor- 
tions ought always to be settled after 
experimenting with the particular coarse 
aggregate and sand which is to be used 
on the job. 

Unfortunately the composition of concrete 
is frequently settled in a very haphazard 
manner. It is not uncommon to find 
specifications in which one part of cement 
is assigned to so many parts of gravel, sand, 
and broken stone, without apparently any 
systematic determination as to whether the 
sand and cement combined together will 
entirely fill the interstices in the larger 
materials. The aggregate should by no 
means be uniform in size, as already ex- 
plained. The average percentage of voids 
in a well-graded aggregate of crushed stone 
is from 36 per cent, to 48 per cent. ; in gravel 
the percentage is a little less, varying from 
30 per cent, to 40 per cent. Sand usually 
contains 40 per cent, to 50 per cent, of 
voids the actual proportion depends on the 
degree of uniformity in the size of the grains. 
From what has been said, the reader will 
know better than to fall into the error of 
supposing that, for example, 1 measure of 
cement, 2 of sand, and 5 of stone will make 
8 measures of concrete. Mixed dry, the 
8f measures would make theoretically only 



26 



REINFORCED CONCRETE 



5 measures, or slightly more, because the 
cement and sand should just fill the voids 
between the stones. 

Determining Percentage of Voids in 
an Aggregate. A method of determining 
the percentage of voids in an aggregate 
is by means of two watertight vessels of 
known capacity ; for the larger vessel, if 
the concrete is for reinforced work, a con- 
venient capacity would be 1 cubic foot ; and 
for the smaller vessel an imperial pint 
measure, there being 49-82 pints in 1 cubic 
foot, which is near enough to take as 50 ; or 
the correct capacity could be arrived at by 
experiment based on the fact that a pint 
of water weighs 20 oz. Fill the larger vessel 
with the aggregate, slightly shake down, and 
strike off level with the top. Should the 
aggregate be of a porous material, it is first 
saturated with water and drained before 
placing in the vessel, as otherwise it will 
absorb water into itself during the measuring 
of the voids, thereby giving a greater value 
for the voids than is actually the case. Now 
pour water into the vessel containing the 
aggregate by means of the pint measure, 
until no more water can be introduced ; note 
the amount required, and take care to do 
the work accurately and without spilling 
any of the water. Frcm the number of 
pints required to fill the vessel in addition to 
the aggregate, the percentage of the voids 
is found by the following rule : 

Number of pints , T 

- x 100, or N X 2, 

50 

N being the number of pints to fill the vessel 
containing the aggregate, and 50 (49'82) 
the number of pints in 1 cubic foot. 

The following is a more accurate method, 
and it involves the use of a galvanised iron 
bucket and a weighing machine. First 
weigh the empty bucket denoting weight 
by W; secondly, weigh the bucket full 
of water, denoting weight by Wj ; thirdly, 
weigh bucket full of aggregate or sand, 
denoting weight by W 2 ; fourthly, weigh 
the bucket full of aggregate and water, 
denoting weight by W $ . Then the cubic 
contents C of the bucket in cubic feet will be 

W W 

C = - , since a cubic foot of water 

oz'4 

weighs 62-4 Ib. The volume of the voids 

W Wo 

V = ^fiiTT^ The percentage of the voids 

Vx 100 
in the sand or aggregate will be 



With sand, it is difficult to drive out with 
water all the air contained in the voids, and 
therefore an error of from 8 per cent, to 10 per 
cent, may be easily made. This can be 
avoided by pouring a measured volume of 
dry sand slowly into a graduated glass vessel 
containing water ; the sand sinks to the 
bottom free from air bubbles, and the 
volume of displaced water may be measured 
and deducted from the volume of the sand, 
the difference being the voids. 

As an illustration of the first method, 
assume that it requires 24 pints of water to 
fill the foot cube (50-pint) vessel of aggregate ; 

24 
then the percentage of voids will be r ,- x 100 

= 48 per cent. By the second method, 
assuming the bucket to weigh 4'12 Ib. = W ; 
when full of water, 41-5 Ib. = W a ; full of 
dry aggregate, 61 Ib. = Wo ; and when full 
of aggregate and water, 79 Ib. = W 3 . 

The volume C of the bucket will be 

41-5 - 4-12 

= (approx.) 06 cubic toot. 



62-4 
The volume of the voids is 



79 - 61 
"62-4 



0-29 cubic foot. 

The percentage of voids will be 

29 X 100 
=48 (approx.). 

Next determine the voids in the sand 
by the same method as for the coarser 
aggregate. Then proportion the cement 
and sand so that the cement paste will be 
10 per cent, in excess of the voids. 

Now, cement when mixed with water 
reduces very much in volume, and it has 
been found that it takes approximately 
100 Ib. of cement to make 1 cubic foot of 
cement paste. It is a simple matter to 
provide 100 Ib. of cement to each cubic foot 
of voids in the sand, allowing an extra 10 
per cent, in excess of the voids, and to pro^ 
portion the volume of the aggregate so that 
all the voids are filled with the cement and 
sand mortar with an excess of 10 per cent. 

One bag of cement (eleven bags to the 
ton) weighs 204 Ib. net, and will make 
2 cubic feet of cement paste. This is a 
convenient basis on which to proportion the 
volume of sand and aggregate for the voids 
to be filled ; the volume of the cement being 
thus fixed as one bag, no measuring or weigh- 
ing of cement will be required, and the result- 
ing concrete will not be too large in volume 
to handle before the first set begins. 



CONCRETE: MATERIALS AND MIXING 



27 



The volume of sand in which the voids 
will be filled with a given volume of cement 
paste is found by the following rule, which 
also answers for finding the volume of 
aggregate to be filled by a given volume of 

C 

cement and sand : ^ - ^-j -- ^. , where C 

v + (ITT X V) 

is the volume of cement paste (or cement 
paste plus sand), and V the percentage of 
voids in the sand (or aggregate). 

Applying this rule to a particular case, 
assume that the voids in the sand are 38 per 
cent, and in the aggregate 48 per cent. 
(That is, taking the bulk of the sand and 
aggregate as 1 each, the voids are re- 
presented by 0-38 and O48 respectively, 
these corresponding to the percentages 
named.) 

The volume of sand in which the voids 
will be quite filled with 2 cubic feet of 
cement paste (made with one bag of cement 

2 
weighing 204 Ib. net) will equal . 



= 4-785 cubic feet = 8,268 cubic 
( '41o 

inches. The internal dimensions of a cube 
having this capacity is arrived at by finding 
the cube root. Thus ^8268 = 20'22 in. 
cube, say 20^ in. = the internal dimensions 
of the sides of the gauge box for measuring 
the sand. 

The volume of cement and sand mortar 
produced is found by Vc + (Vs X (1 P)) : 
where Vc is the volume of cement paste 
in cubic feet, Vs the volume of sand in cubic 
feet, and P the percentage of voids in sand. 
Therefore the volume of mortar produced 
will be 2 + (4-785 X (1--38)) = 4-966 
cubic feet. 

The volume of aggregate for the voids to 
be filled with this quantity of cement and 

4-966 
sand mortar will be 



4-966 

528 



48 + ( T V X -48) 
= 9-405 cubic feet, or 9'405 X 1728 = 



16,251 cubic inches ; for this there will be 
required a gauge box fyl&251 = 25 -33 
inches cube, say, 25 T 5 g- in. in width, depth, 
and length, internal dimensions, for measur- 
ing the aggregate. 

The resulting concrete would be of 
maximum density, after which increase of 
strength would be directly attained by 
increasing the volume of cement paste. As- 
suming that the aggregate had been more 



perfectly graded and the voids had been 
40 per cent., the volume of aggregate re- 
quired for one bag of cement would then have 
4966 4-966 

= : u 28 



.40 + ( T v x -40) 

cubic feet for the same volume of cement 
paste, thus increasing the volume of concrete 
1-885 cubic feet for 2 cubic feet of cement 
paste, and yet maintaining practically the 
same strength. 

The weights of the bags of cement should 
be checked to obviate risk of error. 

Specifying Proportions. In days gone 
by, concrete was specified as 4 to 1, 5 to 1, 
etc., the sand and coarser aggregate being 
measured together, and leading to very 
irregular mixtures and results that could 
not be relied upon. All experienced archi- 
tects and engineers now specify the sand and 
coarser aggregate separately. Thus, a mix- 
ture of 1 : 2 : 4 means 1 part of cement to 2 
parts sand and 4 parts of coarse material 
of various sizes. 

GAUGE BOXES AND MEASURES 

Aggregate Gauges. Gauge boxes con- 
sist merely of four sides fastened together. 
They have no bottom, so that when they 
are filled and levelled off at the top, the 
gauge can be lifted off, leaving the meas- 
ured materials in a heap. They should 
not be made too high, and their capacity 
should not be so great as to make them 
unwieldy. Some form of handle should 
be attached. A good arrangement is shown 
at Fig. 7, where the handles are placed 
midway at the sides. This arrangement 
admits of the gauge box being used either 
end up. Projecting handles at the four 
corners are objectionable as being in the way 
of the workmen, and therefore liable to get 
broken off. The top and bottom edges 
should have strips of hoop-iron nailed along 
them to resist wear. It is convenient to 
make the capacity some definite measure, 
so that it can be used for measuring a cubic 
foot or multiples of a cubic foot, or so that it 
can be used in connection with the scientific 
system of proportioning already described. 
A gauge of J cubic yard capacity is a 
convenient size in general concrete work, 
and if the height is fixed at 1 ft., allows a 
fairly large surface for spreading the cement 
after the gauge has been slightly lifted ; the 
final removal of the gauge is almost the 
equivalent to one turn over. The follow- 
ing example shows how to arrive at the 



28 



REINFORCED CONCRETE 



dimensions of J cubic yard gauge 1 ft. in 
depth : 

iff- ' = 13J ft. super., then v /13^ = 
3 ft. SyV in. nearly. 

So that a gauge 3 ft. 8^ in. by 3 ft. 8 r V in. 
by 1 ft. deep = cubic yard, and a gauge of 




Fig. 7. Gauge Box with Fixed Handles 

the same dimensions top or bottom area, but 
2 ft. deep = 1 cubic yard. Care should be 
taken to make the gauge accurately square, 
and to provide that it shall maintain this 
form, or its capacity may become seriously 
diminished. The joints in the sides should 
be matched or grooved and tongued together, 
and the inside should be rough-planed. 

Cement Gauges. A wooden gauge for 
cement may resemble Fig. 8. It should be 
made as light as possible consistently with 
strength, and furnished with some means of 
transporting it when charged, the cement 
store being often unavoidably some distance 
from the mixing platform. As illustrated, the 
arrangement for conveyance takes the form 




Fig. 8. Cement Gauge 

of two loose carrying bars, fitting under 
wrought-iron clips or brackets, fixed to the 
cleats at the sides with stout ironwood 
screws. This gauge should be accurately 
square, and have a bottom to it. 

Metal gauges for cement are lighter and 
handier than wooden ones ; but cement 
gauges are not necessary when the whole 
system of proportioning is based on the 
weight of cement contained in one bag. 
The best method of apportioning cement is 
by weight, and not by cubic measure. 



Water Gauges. If the water is not 
measured accurately, defective concrete will 
be produced. Odd buckets and cans should 
be avoided as tending to errors, which, 
occasionally, become very serious, and only 
those measuring appliances should be used 
that have been carefully examined and certi- 
fied by some responsible person. For small 
quantities of water a conveniently-sized can 
or bucket, as tall and narrow as possible, may 
be used, providing that it has had its exact 
capacity or some distinguishing mark legibly 
painted on it. For larger quantities, a cistern 
holding just the amount o water required 
for one batch of concrete should be placed 
above the mixing board or machine. This 
cistern should be provided with an outlet pipe 
and tap as near to the bottom as can be 
arranged. A short piece of rubber hose 
attached to the outlet pipe is often a great 
convenience. All buckets, cans or cisterns 
should have, legibly painted inside them, a 
mark to which they are to be filled. This 
mark should be as near as possible to the top 
of the vessel, as the risk of adding too much 
water is thereby reduced. If the quantity 
of concrete is very large, it is more con- 
venient and accurate to have a cistern fitted 
with an overflow pipe, and to run water 
into the cistern until it begins to overflow ; 
the disadvantage of this arrangement is that 
it is troublesome to adjust the overflow pipe 
in the first instance. Some firms have found 
tanks operated by siphons to be convenient 
and accurate ; a cistern controlled by a ball- 
cock is used to fill the measuring tank, and 
the latter is emptied siphonically with such 
suddenness that the amount of water intro- 
duced by the opening of the ball-cock is too 
small to cause any appreciable error. Where 
variable quantities of concrete are to be 
mixed, a cistern with its capacity at different 
levels painted on it should be used. All 
the water to be used for one batch should 
be held at one time in the water gauge. 

MIXING CONCRETE BY HAND 

There are but few hard-and-fast rules 
with regard to the mixing of concrete. 
Most foremen and clerks of works favour a 
particular system of their own, and as long 
as the result in all cases is a properly mixed 
material the quickest method is the best. 
On no account must the concrete be worked 
after setting has begun. In a method that 
was common until a few years ago, the 
materials were measured out (on a 10 ft. 



CONCRETE: MATERIALS AND MIXING 



29 



square platform) in a heap on the top of 
one another, the cement being added last 
from a wooden box. The heap was then 
turned over with shovels, one man having 
an iron prong ; the water was applied gently, 
so as not to wash the cement away. The 
concrete was turned often enough to obtain 
a thorough incorporation of the ingredients. 

In another method, the aggregate was 
placed in a layer from 8 in. to 12 in. thick 
over a platform, the smaller pebbles being 
at the bottom. The cement was then spread 
as uniformly as possible over the whole, the 
materials being then mixed by four men, 
two with shovels and two with hoes, the 
former facing each other, and always working 
from the outside to the centre, then stepping 
back and going over it again in the same 
way ; the operation was continued until the 
whole mass was turned. The heap was then 
turned over again in an opposite direction, 
the surface of every pebble thus being 
covered with cement. Two turnings usually 
sufficed to make the mixture complete. 

A method that once had the approval 
of engineers is to spread out the sand over 
the platform to the depth of a few inches, 
and over this to spread the cement. A hole 
is made in the middle by means of a hoe, 
and the water poured in, the whole being 
then mixed by means of shovels and hoes 
to form a thin paste. The aggregate straight 
from the washing, or purposely wetted, is 
added, and the whole mixed. This is a bad 
method ; unless the mixing board has a 
fillet all round it, there is a risk of losing 
much of the fine stuff, and the method is 
slow, giving rise to the temptation to 
scamping. 

A better method is first to spread out the 
aggregate, then the sand over it, and lastly 
the cement over the sand. Two shovellings 
will then mix the material fairly well. It 
is usual to measure the aggregate in a gauge 
box, smooth out the aggregate so that the 
sand gauge may go on top of it, and then fill 
the gauge with sand. In some cases a slight 
allowance is made for the sand which will 
fall into the voids of the aggregate, but this 
is scarcely necessary if the top of the heap 
of aggregate is smoothed fairly well. Lastly, 
the cement is added from a wooden or metal 
measure, or possibly straight from the bag. 
Preferably, the water should be supplied 
through a rose, as the more gentle the 
application the better. The water is applied 
during a third shovelling, following which 



may be a fourth, or the concrete may be 
shovelled direct into the barrows or other 
conveyers. 

The Association of American Portland 
Cement Manufacturers recommends the 
following method : the sand is spread over 
the board in a layer 3 in. or 4 in. thick, and 
over it is spread the cement. Two men start 
mixing the sand and cement, each man 
turning over the half on his side, starting at 
his feet and shovelling away from him. In 
turning the shovel, the materials must fall 
off the end and sides so that the materials 
are mixed as they fall. The mixed sand and 
cement are spread out carefully, and the 
gravel or stone measuring box is placed 
beside it, filled, lifted off, and the gravel is 
shovelled on top of the sand and cement, 
spreading it evenly. With some experience 
equally good results can be obtained by 
placing the gravel measuring box on top of 
the carefully levelled sand and cement 
mixture, and filling it, thus placing the gravel 
on top without an extra shovelling. Add 
about three-fourths the required amount 
of water, using a bucket and dashing the 
water over the gravel on top of the pile as 
evenly as possible. The men turn over the 
materials in much the same way as they 
did the cement and sand, except that, instead 
of shaking the materials off the end of the 
shovel, the whole shovel load is dumped and 
dragged back towards the mixer, so that 
the wet gravel picks up the finer material. 
Water is added to the dry spots as the mixing 
proceeds until the allowed quantity has been 
used. The mass is turned over again, and 
if it is streaky or shows dry spots it must be 
turned again. After the final turning it is 
shovelled into a compact pile. 

"Dry" Mixing. The so-called "dry" 
method of mixing is of interest to the concrete 
block maker who desires to remove the 
shaped block from the block-making machine 
with as little delay as possible. In the 
United States, too, it is used in cases where 
the contractor is working on a time limit, 
and wishes to remove the centering, etc., at 
the earliest possible moment. By this 
method, no more water is used than will hold 
the ingredients together when some of the 
material is taken in the hand and squeezed. 
Even such concrete as this will show moist 
on the suiface after tamping. The concrete 
attains its strength more quickly by this 
method than when it is made really wet, but 
there appears to be no difference in results as 



REINFORCED CONCRETE 



regards strength when samples of the two 
kinds are tested after an interval of a couple 
of years. 

MACHINE MIXING AND MIXERS 

The popularity of reinforced concrete has 
been the underlying cause of the attention 
now being paid to the design and construc- 
tion of concrete mixers. The necessity of 
producing a concrete of high quality, and 
in which the whole of the surface of the 
aggregate is coated with cement, has had 
the effect of directing inventive faculty 
towards the provision of a machine to do 
the work more efficiently than is possible 
when shovels are used. A certain knack is 
necessary in hand mixing, and it is not all 
labourers that possess it ; then, too, the 
mixing is heavy work, and is likely to be 
scamped when opportunities for doing so 
arise. The introduction of the mechanical 
mixer was the occasion for much con- 
troversy, which has long since been settled 
in its favour ; only now and then, as in the 
case of an important building at West- 
minster erected in 1910-12, do these re- 
sponsible prefer hand mixing. 

In selecting one of the forty mixers obtain- 
able, the question is not so much nowadays 
whether it mixes concrete efficiently, but 
how long the operation takes. To the time 
employed in the actual mixing must be 
added that for charging and discharging, 
but there is no doubt that the employment 
of any of the well-known machines now 
obtainable is sure to save time and money. 
Machine mixing undoubtedly exercises 
economy in the most costly of the three 
materials namely, cement inasmuch as a 
given amount can be incorporated efficiently 
with a larger amount of sand and aggre- 
gate than is possible with hand mixing. 

Choosing a Mixer. The concrete mixer 
has to be designed and constructed to work 
under very severe conditions, the mention 
of some of which will suggest to the practical 
man the points to be borne in mind when 
choosing a machine. It must be simple, and 
have as little mechanism as possible, as it 
will be tended, not by mechanical engineers, 
but chiefly by labourers, and any delicate 
mechanism would soon be clogged by the 
cement. It must be strong, because it has 
to accommcdate heavy batches, and the 
metal in contact with the concrete must 
be thick and hard, since the aggregate has 
great abrasive properties. All parts sub- 



ject to wear by the concrete should be easily 
renewable, and the makers should be asked 
whether or not they can replace from stock 
the parts in question. It must be compact 
and eminently portable, because it may have 
to be used on a number of floors in the same 
building, and thence may require to be carted 
miles away to another job. In its design 
there must be a complete avoidance of 
arrangements which theoretically are good 
but practically are bad ; in other words, 
the machine needs to have been designed 
by someone with practical experience in 
mixing concrete, as the problem is a different 
one from that which the designer of mixing 
machines for soft goods is required to solve. 
Not only the actual mixing, but all the 
accessory processes must be executed 
speedily ; it must be easy to charge the 
machine, and a matter of a few seconds to 
empty it. It must do its work thoroughly, 
by which is meant that the cement must 
be uniformly distributed throughout the 
sand and aggregate, every particle of which 
must be coated. 

Concrete is not an easy material to mix, 
each of the three ingredients presenting 
difficulties of its own. The cement is liable 
to bridge in a tapered hopper ; its dust flies 
about and clogs any mechanism present ; if 
the machine is not cleaned out regularly the 
cement will be retained in angles and 
corners ; and a moist cement mixture does 
not work freely, the trouble increasing as 
the proportion of cement increases. It 
might be thought that if cement and sand 
were run into a revolving drum and this 
speeded up to a high velocity, the maximum 
of efficiency would be obtained, but it is 
not so, as at a certain velocity the cement 
flies to the side and clings there. Sand, if 
at all damp, refuses to run unless the shoot 
or hopper is steeply inclined (experience 
shows that the inclination must be at least 
30 degrees to the vertical). The difficulty 
in dealing with the aggregate is, first, its 
abrasive action, which rapidly wears out 
soft metal ; and, secondly, the trouble caused 
by stones becoming wedged between two 
rotating arms or between an arm and the 
side of the mixing vessel ; the latter trouble 
is not apparent in most of the tumbling 
barrel mixers. 

Types of Mixers. Coming now to 
details, two broad types of mixer are at 
present in use, respectively (1) the batch or 
intermittent mixer, which mixes at one 



CONCRETE: MATERIALS AND MIXING 



time any quantity up to the full capacity 
of the mixing vessel ; generally it is a 
rotating drum, to the inner sides of which 
are riveted blades which assist the mixing 
action ; a lesser-used type has a pan, either 
revolving or stationary, with paddles or 




Fig. 9. Sectional Diagram showing Principal 
of Chicago Cube Mixer 

stirrers to agitate the material. (2) The 
continuous mixer, which delivers a con- 
tinuous stream of mixed concrete ; the most 
popular type consists essentially of an 
inclined cylinder with suitable mixing or 
agitating appliances. 

The method of measuring or gauging the 
materials is of the greatest importance in 
connection with machine mixing. In the 
case of the batch mixer the measures are 
boxes, hoppers, or even the wheelbarrows 
in which the material is conveyed to the 
machine ; but the most convenient form 
of measurer is a slap which may be lowered 
to the ground to receive the charge, and 
then in a few seconds elevated and tilted 
so that its contents pour into the mixing 
drum. 

As regards continuous mixers, owing to 
the nature of the mixing process the 
materials have generally to be measured 
and roughly incorporated in their dry state 
before they are introduced into the machine ; 
or, as an alternative, there is, as a part of 
the machine, the necessary mechanism for 
proportioning the ingredients. Examples of 
machines so equipped are illustrated later. 

There is a growing consensus of opinion 
in favour of the batch mixer for reinforced 
concrete work, while the other type is found 
to have advantages in connection with large 
works, such as harbours, coast defences, etc., 



where the concrete requires to be deposited 
in bulk, and where, possibly, immense 
quantities are required in one place. The 
preference given to the batch mixer is due 
to a number of reasons : (1) It lends itself to 
the intermittent nature of the work. (2) It 
is thought that the clerk of works can exer- 
cise a more careful supervision over the pro- 
portions of the ingredients, but whether as 
a matter of fact he can do so in practice is 
a moot point, as obviously in a building of 
any size he has other things to do than 
watch a mixer at work for several consecu- 
tive hours. (3) Both the proportions of 
the ingredients and the duration of the 
mixing can be easily varied, whereas it is 
a matter of difficulty in some continuous 
mixers to make any appreciable alteration 
in the duration of the mixing without inter- 
fering with the efficiency of the product, as 
any slowing down or speeding up of the 
machine often has the effect of necessarily 
altering the proportion of water admitted 
to the mixture. The continuous mixer has 
a different mixing efficiency with different 
classes of aggregate, and, theoretically, with 
any increase in the proportion of sand the 
inclination of the cylinder should be in- 
creased, this not often being possible. (4) 
The batch mixer enables the ingredients to 
be mixed dry, if required. 




Fig. 10. Chicago Cube Mixer with Engine 
and Boiler 

The power for driving a mixer may be 
either hand or engine, and in the majority 
of cases the larger mixers are self-contained, 
a boiler and engine, an oil engine or petrol 
motor, or an electric motor to be connected 
up to the supply mains, being mounted on 
the same base as the mixer. Naturally, the 



REINFORCED CONCRETE 



power-driven machine has a much greater 
capacity than the hand-driven one. A 
warning with regard to advertised capacities 
may here be given. Makers often state most 
definitely the weight or the number of cubic 
feet of concrete which their machines can 
mix in the course of an hour, and they 




Fig. 11. "Cut-away" View of Eclipse 
Mixing Drum 

advertise the results of perfectly honest 
tests ; but inasmuch as the capacity of a 
machine must vary according to individual 
conditions the disposition on the site of 
the cement, aggregate, etc., the nature of 
the job, and the amount of experience of 
the operatives a contractor who is choosing 
a machine should not place too much reliance 
upon any comparison he may draw between 
two sets of figures. 

BATCH MIXERS DESCRIBED 

Revolving Drum Machines. There is 
any number up to a score of these from 
which to choose. 

Chicago Cube Mixer. This is based essen- 
tially on the old-style tumbling drum a 
cubical box journalled at diagonally opposite 
corners but the shaft in the old-style mixer 
is replaced with hollow trunnions riding on 
rollers and made large enough to serve as 
openings for charging and discharging the 
mixer (see Fig. 9). To rotate the cube 
there is a circumferential rack fastened 
around it on a drum at right angles to, and 
midway between, the trunnions ; this rack, 
geared with a pinion shaft, is operated by 
the engine shaft in such a way that there 
is no gearing in the way of the materials 
either in charging or discharging. The 
sharp corners and edges of the cube are 
rounded off to obviate the possibility of the 



fine material effecting a lodgment. The 
manufacturers of this mixer have found that 
the batch is folded over on itself, and pressed 
into a contracting space shaped alternately 
like a wedge and like a pyramid ; and on 
this fact is based their statement that the 
mixing is done by kneading and not by 
stirring. Fig. 10 shows the cube mixer, 
with steam engine and boiler ; another type 
has a petrol motor. 

Barker and Hunter. The special feature of 
this machine is that it cannot be used as a 
continuous mixer. It is fed while revolving 
in one direction, and must be reversed before 
the mixture can be discharged. The actual 
mixer consists of two truncated cones, whose 
bases merge into a cylinder, the ends of the 
cones being mounted with roller path bear- 
ings and rotating on rollers mounted on a 
suitable frame. Two hoppers are fitted, and 
the mixer can be charged from either end or 
from both ends, which are always open, with 
the cone either stationary or in motion. 
The makers say that it takes twenty seconds 
to charge the material into the cone, sixty 
seconds to mix them completely, and ten 
seconds to discharge. 

Eclipse. This machine is built shallow 
so as not to necessitate mechanical charging 
devices. The drum has a shell of rolled 
steel plate, and has flanged steel heads ; it 
is supported by trunnion rollers, and is 
rotated by gears meshing into two gear 
rings. At the charging end is a large cir- 
cular opening in the flanged head, and 




Fig. 12. Eclipse Mixer with Petrol^Motor 

diagonal overlapping charging blades extend 
into the drum. In the interior of the drum 
are diagonal blades firmly attached to the 
sides. One blade extends from the charging 
end diagonally to the discharging pocket 
shown at the rear of the drum in Fig. 11. 
The aggregates are carried into the mix- 
ing chamber by conveying blades rigidly 



CONCRETE: MATERIALS AND MIXING 



33 



attached to the sides and ends, so designed 
that the material cannot slop out when 
mixing. The discharging shoot extends 
into the drum through the head at the dis- 
charging end. A shaft extends through the 
drum near the shoot, and on it is mounted 
a door held during either mixing or dis- 
charging by a strong spring under tension. 
When the operator throws the door past the 
centre of tension in either direction, the 
spring completes the movement and holds 
the door until it is again held by the operator. 
Fig. 12 is a special design mounted on a 
truck and equipped with a petrol motor 
housed in the casing. 

Gauhe. In this mixer the drum (supported 
by friction rollers) revolves, while paddles 




Fig. 13." Cut-away " View of Koehring 
Mixing Drum 

and a scraper are fixed and serve to mix 
the material. The material enters the drum 
through an opening at one end, and the 
discharge is through a sliding door controlled 
by a lever. The elevating device is on the 
lines of the Fawcett, described later, and 
therefore, like that machine, it requires con- 
siderable head-room. The road wheels are 
of large diameter, so as to allow trolleys to 
pass beneath the mixing drum to receive the 
concrete. The equipment includes an auto- 
matic water-measuring tank. 

Koehring. In this the drum is of boiler- 
plate steel with cast heads, surrounding each 
of which is a gear ring. The double drive 
has the advantage that, should a tooth 
break, the machine can continue on a single 
drive. An interior view of the drum is 
given in Fig. 13, whilst Fig. 14 is a photo- 
graph of the mixer with steam boiler and 

3 



engine, side loader, and water measuring 
tank (shown above the mixing drum) ; the 




Fig. 14. Koehring Mixer with Engine 
and Boiler 

operating levers and hoisting mechanism are 
clearly shown. The discharge shoot is made 
in two pieces, the outer end being stationary, 
and the inner end pivotal, so affording ample 
clearance for wheelbarrows in the discharge, 
and permitting the inner end of the shoot 
to assist in the mixing when tilted inwards. 
The side loader is a short, wide, and low 
bucket with round corners and sufficiently 
low pivotal point to require but little power 




Fig. 15. Marsh-Capron Non-tilting 
Mixing Drum 

to raise it ; it can be elevated to almost a 
vertical position, so that the material readily 
flows into the drum. 



34 



REINFORCED CONCRETE 



MarsJi-C apron. Both tilting and non- 
tilting machines of this manufacture are 




Fig. 16. " Cut-away " View of Marsh-Capron 
Tilting Mixing Drum 

known, and in each the drum is the special 
feature. In the non-tilting style, the drum 
(see Fig. 15) is cast in two parts, the con- 
necting flanges fitting one within the other 
and centering the two halves, which are 
securely bolted together. The steel mixing 
blades are bolted to the drum, and both 
these and the steel mixing buckets are so 
placed as to give a thorough mixing action 
to the material, providing an end-to-end as 
well as a lifting and pouring action. The 
driving gear surrounding the drum consists 
of five segments absolutely interchangeable, 
so that should a tooth be broken a segment 
can be renewed at the labour of removing 
and repl icing five bolts, 
the drum being kept in 
place 'and the rest of the 
machine remaining undis- 
turbed. In the tilting 
machine the drum (see 
Fig. 16) has its blades 
arranged in serpentine 
form and attached with 
brackets, there being 
sufficient space between 
blades and drum to allow 
of thorough washing out. 
Fig. 17 shows the machine 
with the drum in the 
tilting position. 

McKelvey. In this ma- 
chine the drum consists Fig. 17. Marsh 



of a short cylinder with a conical hopper 
on the feed side and a trumpet-shaped 
discharge funnel on the other side. It re- 
volves on friction rollers, and is operated 
by a chain and sprocket ring. Its chief 
feature is the means adopted for ensuring 
the proper mixing of the material. The 
special device used (see Fig. 18) is known 
as a " gravity shovel," and is pivoted to the 
inside of the drum ; the top edge lies against 
the rising side, collects the materials in that 
part of the drum, and, after it is filled, the 
overflow slides rapidly of? the lower edge 
and the balance is carried upward to a point 
where gravity causes the shovel to cast the 
material outward and downward, turning 
it over in the fall. 

Messent. This is based on the old tum- 
bling barrel, the design being a closed vessel 
of irregular shape revolving on an axle. 
As used on the Dover Harbour works, it 
was mounted on a steel-framed carriage, 
there being one electric motor to rotate 
the mixing vessel and another to give a 
travelling motion to the carriage, the 
materials being thus mixed in course of 
transit between place of charging and the 
work. 




-Capron Mixer, with Drum in tilting position " 



CONCRETE: MATERIALS AND MIXING 



35 



Pioneer. This is a small hand mixer, 
capable of handling between 250 Ib. and 
350 Ib. per batch. It can easily be worked 
by one man, and is too simple to require 
detailed explanation, being simply a mixing 
drum supported on a horizontal spindle. 
The materials are introduced, the lid closed, 




Fig. 18. McKelvey "Gravity Shovel" 

the drum rotated three or four times, then 
the water contained in the tank at the side 
is allowed to enter the drum, through the 
perforated shaft, and the drum is finally 
rotated again a few times, the mixture then 
being complete. 

Ransome. Machines of this make largely 
owe their conception to E. L. Eansome, 
whose name is of importance in the history 
of reinforced concrete in the United States. 
A variety of Ransome machines is made, 
all incorporating the vital feature namely, 
a drum with mixing blades of a special type. 
Figs. 19 to 24 are photographs of six 
arrangements of the Ransome mixer, there 
being an explanatory inscription to each 
figure. The drum is not of the tilting type, 
the discharge being made possible merely 
by opening a shoot. Each type of machine 
has a hopper which measures the exact 
amount of aggregate for each batch, there 
being in addition a water tank which 
automatically gauges the exact amount of 
water required. Within the drum is a series 
of steel scoops (see Fig. 25) which plough 
through the material repeatedly, pick it up, 
and carry it upwards till it slides out. The 
aggregate having been delivered from the 
hopper into the drum, the hopper gate is 
closed and the hopper re-charged in readi- 
ness for the next batch. The elevating 
hopper is so arranged that it can be lowered 
until its bottom rests on the ground. When 
required, a lever is pushed, and the elevator 
tips the material into the hopper. Fig. 27 



shows two elevations and a plan of a 
Ransome belt-driven mixer. 

In Great Britain the Ransome mixers are 
built by Ransomes and Rapier, Ltd., of 
Ipswich, and in the United States by the 
Ransome Concrete Machinery Co., Dunellin. 
The English and American machines con- 
form to the same general patterns, but there 
are minor differences of detail ; for example, 
in the American mixing machine the interior 
of the drum resembles Fig. 26. 

Roll. This has distinct points of novelty 
(see Fig. 28). The drum consists of two 
cup-shaped halves mounted on a spindle 
in such a way that they can be drawn apart 
to discharge the concrete. The drum con- 
sists of two castings, to one of which is 
bolted a sleeve carrying a worm which 
serves to move one half of the drum along 
its spindle when discharging is necessary. 
The mixer is equipped with an elevator 
consisting of a measuring skip hauled up 
by a wire rope and returning by gravity. 
Smith Hand Mixer. As shown by Fig. 
29, this machine is driven by means of 
crank handles one man to each side but 
it is easily adapted for power driving. The 
ends of the drum are sloped inwards until 
they almost meet, forming two wedge-like 
chambers united by a 4|-in. slot, extending 
diametrically across the drum. The drum 
itself is suspended on chains, four guide 
rollers being provided to keep it in align- 
ment and prevent it from swinging during 
the mixing process. The principle of the 
mixing can be understood by bearing in 
mind the old-fashioned hour-glass, the 
material from one chamber having to pass 
through a restricted opening into the other 
one. Only two or three turns of the drum 
are required. The charging platform sup- 
plied is only 15J in. high, thus enabling a 
wheelbarrow to charge directly into the 
machine, and the mixer is so constructed 
that, if it were desired, it could discharge 
directly into a trench. 

Smith Power Mixer. The drum of this 
machine consists of two cones united at 
their bases, as shown in Fig. 30. The 
blades which assist the mixing are arranged 
spirally ; and at the discharge end are drip 
rings which prevent the wet material from 
running down the cone and over the roller 
tracks. The drums are partially lined ; for 
instance, the large ends of the cones are 
made of double thickness, because that is 
where the principal wear comes. This 



REINFORCED CONCRETE 



lining is a regular repair part and is easily 
fitted. With certain of the sizes a power 



having a double conical drum which is tilted 
to discharge the concrete. 




Fig. 



19. Belt-driven Ransome 
Mixer 



Fig. 20. Ransome Mixer 
with Hoist 



Fig. 21. Ransome Mixer 
with Elevating Hopper 






Fig. 22. Ransome Mixer 
with Electric Motor 



Fig. 23. Ransome Mixer 
with Boiler and Engine 



Fig. 24. Ransome Mixer 
with Oil Engine 



tilting apparatus is supplied, the makers 
believing that the tilted drum provides the 
fastest method of discharging. It is only 
the smaller sizes of the Smith power mixer 



Victoria. The drum of this mixer is a 
cylinder with four deflecting surfaces or 
inclined planes (see Fig. 33). Lifting 
blades are riveted to the drum and extend 




Fig. 25. Scoops in Ransome Mixing Drum 
(English) 

that are recommended for reinforced con- 
crete work. 

Taylor. This resembles the Smith in 



Fig. 26. Scoops in Ransome Mixing Drum 
(American) 

from the discharge end well towards the 
feed end. As the drum rotates, the lifting 
blades elevate the material, which drops 



CONCRETE: MATERIALS AND MIXING 



37 



upon the inclined planes and is thus thrown 
across the drum and returned again by the 
opposing deflectors. The process is repeated 



the spout is inserted into the drum the mass 
falls from the blades upon it, and the con- 
crete flows into the receptacle prepared for 




twice for each revolution, and submits the 
mass to twelve distinct mixing actions per 
turn of the drum. The complete machine, 
on a truck, with engine and boiler, side 
loader, and automatic water tank, is shown 
in Fig. 31. There is a low feed level, which 
is of especial advantage in a portable plant 
charged by wheelbarrows. For the purpose 
of emptying the mixture, a swinging spout, 
pivoted below the discharge opening, is pro- 
jected into the drum. It will be understood, 
on referring to Fig. 31, that the spout is 
pivoted to the vertical frame shown, and 
easily swings forwards or backwards. When 




c _ _ 



Fig. 27, Elevations and Plan of Ransome 
Belt-driven Mixer 




Fig. 28. Roll Mixer, with Elevator raised 



REINFORCED CONCRETE 



it. The flow is under easy control, as the 
shoot may be removed from the drum at 
any moment by a mere turn of the wrist. 
Any quantity, from a quart to the full mixing 
capacity, can be withdrawn. The charging 
skip is shown in the elevated position in 
Fig. 31, and in its bottom position in Fig. 
32. This skip will hold a complete batch, 
and, as it progresses upwards on the curved 
guides shown, its nose is thrust into the 
feed opening of the drum. Care has been 



of concrete ; for this purpose the gauge is 
set for the maximum capacity, the tank is 
allowed to fill, and the valve is then opened. 
The pointer on the outside will then fall, and 
a glance at the gauge will show at any instant 
how much water has run out. The necessary 
water for a batch in a large-size mixer is 
best obtained by measuring the water into 
the mixing drum in two lots ; this is safer, 
as a rule, than introducing all the water at 
one time. When this automatic water tank 




Fig. 29. Smith Hand-driven Mixer 



taken to make the feed angle steep to pre- 
vent clogging, and to increase the rapidity 
of the feed. The makers provide extensions 
for the loader frame when required, so that 
the skip can be lowered to receive the batch 
on a level below the mixer. In the two com- 
plete views a rectangular tank will be noticed 
at the top. This is an automatic water 
tank of 18-gal. capacity, with a gauge which 
can be set so that the tank will supply 
uniform quantities of water varying from a 
few drops up to the full capacity of 18 gal. 
The tank serves as an indicator to show the 
amount of water introduced into any batch 



is a part of the equipment, it is necessary 
to supply the water to it under pressure, or 
from a reservoir placed at a higher level 
than the tank. The makers fit either steam 
engine, petrol engine, or electric motor to 
their mixers, and they particularly recom- 
mend the slow-speed electric motor which 
may be geared direct to the mixer. 

Other Batch Mixers. The drum mixer 
is not the only type of batch machine. There 
are a few examples of mixers having pans 
and stirrers. 

Express. This has a stationary pan to 
hold the material (see Fig. 34), revolving 



CONCRETE: MATERIALS AND MIXING 



39 



in it being a series of paddles and rakes 
attached to arms which project from a central 
power-driven capstan head. By means of 
two discharge doors in the bottom of the 
pan the smaller size mixer can be emptied 
in fifteen seconds, both feeding and dis- 
charging taking place while running. 

Faivcett. The material is contained in a 
fixed drum in which a number of arms or 



filled, ascends along a pair of inclined guides 
by means of a wire rope which is wound on 
a drum actuated through a friction wheel. 
When the skip arrives at the top, it tips 
automatically into the mixing vessel and is 
then lowered for another charge. 

Gaspary. The hand-driven mixer (Fig. 36) 
has a trough capable of treating up to 45 
cub. ft. an hour, according to size. Above 




Fig. 30. Smith Power-driven Mixer 



paddles rotate in different directions, the 
arrangement being such that when the dis- 
charge door at the bottom of the drum is 
opened, the revolving paddles scrape the 
concrete out (see Fig. 35). The mixing is of 
an efficient character, and the open pan 
enables the attendant to see when the con- 
crete has attained an even colour through- 
out. The machine has an elevator for 
measuring and feeding the charge, and an 
automatic water tank for measuring the 
water. The elevator is a skip which, when 



the tilting trough is a small water tank, the 
water reaching the materials through a per- 
forated pipe. In the power-driven mixing 
machine, there is a drum with blades 
which rotate in either direction, this change 
in the direction being of advantage for 
cleaning and emptying the mixing drum, 
and allowing of any stones stuck fast be- 
tween the drum and the blades being in- 
stantly removed. Continuous machines are 
also made by the Gaspary firm, as noted 
on a later page. 



4 o 



REINFORCED CONCRETE 




Fig. 32. Victoria Mixer, with Skip Lowered 



Oehler. This is a Swiss machine - 
resembling the Fawcett, but the 
paddles rotate in one direction only. 
Open - Drum. This mixer has 
a cylindrical drum, normally up- 
right, with conical top which 
facilitates discharging. The sides 
of the drum are of steel plate, 
but the bottom is a casting having 
a circumferential rack, by means 
of which the drum is rotated 
through gearing. The drum is 
carried by a supporting frame, in 
which is a ball-bearing for a 
short vertical shaft which pro- 
jects into the mixing drum. The 
mixing is efficient, and is much 
facilitated by parallel blades 
riveted to the sides of the. drum. 
The drum, being so near the ground, 
can easily be charged from wheel- 
barrows on a loading platform, 
while for discharging there is a 

hand or power arrangement for tilting the Pansy. As shown in Fig. 37, there is 
drum. The capacity varies from 2J to 18 an annular vessel which runs on a ball- 
cub, ft., according to the size and style. bearing cast-steel base, and is driven through 

toothed gearing. Through the 
centre of the machine is a per- 
pendicular shaft, to which are 
hung angle-iron frames, and to 
these again adjustable ploughs 
are fixed. The pan revolves at 
the rate of ten or twelve revolu- 
tions per minute, and as the 
ploughs remain stationary, the 
materials are thoroughly mixed 
in a short time. The materials 
are gauged in the ordinary way 
on a platform above the ma- 
chine, and tipped directly into 
it, being there levelled by a 
scraper, the ploughs lowered and 
the material mixed dry. Next, 
water is added, and the materials 
further mixed. When mixing 
is complete, the ploughs are 
raised, the scraper lowered, and, 
by means of the trapdoors in 
the bottom, the mixed concrete 
passes into a receptacle prepared 
for it. The doors close auto- 
matically by means of a special 
catch, and as soon as the 
material is all discharged the 
pan is ready for another batch. 
Whalley. In this machine the 
Fig. 31.-Victoria Mixer, with Skip Elevated materials are discharged from a 




CONCRETE: MATERIALS AND MIXING 



hopper (see Fig. 38) into a stationary pan 
and the mixing is done by stirrers 
mounted on arms projecting from a verti- 



for hand use, and which may be either 
movable or stationary according to require- 
ments. The materials have to be fed in 





Fig. 33. " Gut-away " View of Victoria 
Mixing Drum 

cal shaft. Each stirrer comprises a vertical 
cutting part and a scraping part, the latter 
being almost in contact with the bottom 
of the pan. The stirrers 
tend to force the mate- 
rial towards the sides, 
and when the mixing 
has been completed a 
door is opened and the 
mixture discharged sim- 
ply by the continued 
rotation. The parts of 
the machine liable to 
wear or replacement are 
the bottom portions of 
the stirrers, and they can 
be readily renewed. 

CONTINUOUS MIXERS 

N o n - p r o portioning 
Machines. These are 
of much simpler con- 
struction than the mixers 
fitted with proportioning 
or measuring arrange- 
ments, and a few exam- 
ples will now be briefly 
described. 

Gaspary. Only the 
smallest of the Gaspary 
continuous mixers need 
be mentioned here. Fig. 
39 shows a power-driven 
machine, which, how- 
ever, can be adapted 



Fig. 34. Express Mixer 

measured quantities, the water being 
obtained from a supply tank shown above 
the mixing cylinder, there being a perfor- 
ated pipe extending from the tank right into 
the cylinder. The dry mixture enters the 
cylinder, is wetted and mixed, the mixing 
action being assisted by shovels which are 
turned by the rotation of the cylinder. 

Mason. The feature of this machine is 
a cylinder slightly larger in diameter at one 




Fig. 35. Fawcett Mixer 



REINFORCED CONCRETE 



end (the discharge end) than at the other 
where the feed hopper is situated (see 
Fig. 40). The cylinder is mounted on a 




Fig. 36. Gaspary Hand-driven Tilting Trough 
Mixer 

central spindle and has ribs to assist the 
mixing. Machines for hand or power are 
made. 

Gravity Mixers. No power is required in 
the operation of a gravity mixer, the force 
of gravity alone being relied upon to do the 
work. The materials are shovelled into the 



number of baffle plates. The rows of pins 
are staggered with respect to the rows 
immediately above and below them, and 
it follows that when the materials fall from 
the hopper on and off the pins and deflectors, 
they become incorporated. The water is 
added (by means of a flexible hose) half-way 
down, this giving the materials a chance to 
get partly mixed before being wetted. 

The Owens is a gravity mixer in which, 
there is a steel shoot about 7 ft. long con- 
taining three sinuous mild steel bars extend- 
ing in one length from the top to the bottom. 
The bars are fixed rigidly at the top, but at 
the bottom are held loosely by means of a 
|-in. pin, which passes through them. In 
addition, there are eight large and six small 
baffle plates fixed to the sides of the shoot. 
To facilitate cleaning, there is an inspection 
door. The water supply pipe is fixed behind 
the lower edge of the top baffle plate, and 
is perforated both back and front, the 
whole of the baffles and bars receiving a 
spray of water. The materials, having been 
introduced in any convenient manner, fall 
through the shoot and are alternately split 
up into two columns and then united, 
this action being repeated eight times. 

Proportioning or Measuring Machines. 
Brief particulars will now be given of 
eight examples of continuous mixers with 
provision for proportioning the concrete 
ingredients. 

Bolte. This is an American machine (see 
Fig. 41) adapted for hand or power, accord- 




Fig. 37. Pansy Mixer 



top end of a vertical or inclined shoot, a 
common pattern of which has a large num- 
ber of round pins penetrating the shoot from 
front to back, there being in addition a 



ing to size. The principle of this machine 
is the conveying of proportioned quantities 
of cement and aggregate, by means of a 
rubber belt, into a mixing cylinder, where 



CONCRETE: MATERIALS AND MIXING 43 

the material is agitated by six blades. The may be taken as representative. It has 
hoppers are shown in Fig. 41a. The belt three hoppers or pockets for automatic- 
forms the bottom to them and draws off ally proportioning aggregates, sand, and 

cement, and the proportioning 
device works without the use of 
gears, sprockets, or chains. The 
pockets .on each side are oper- 
ated by one sliding belt, which 
moves forward and backward on 
rolls and acts as a plunger for 
delivering the material. The 
sliding part is moved by two 
crank arms attached to the square 
shaft, which also passes through 
the cement hopper, where its 
reciprocating action prevents any 
bridging of the cement. Each 
material can be proportioned to 
a nicety, and the materials can 
be fed in from one or both sides. 
The mixing device is a special 
feature, and unlike any other 
mechanism described in these 
pages. It consists of seven steel 
blades twisted spirally and bolted 




Fig. 38. Whalley Mixer, with Engine and Boiler 



at each end to a solid casting, 
as shown in Fig. 43. This con- 
struction cuts through, lifts, and 

the materials through adjustable gates, pours the material, the result being a 
discharging them into the cylinder, where, most efficient mixing. Between the engine 
15 in. from the lower end, the water is intro- 
duced by means of a perforated pipe con- 
nected to the water tank above. As it is 
well known that cement and sand do 
not readily flow at 
all times, the hop- 
pers are fitted with 
agitators, the cement 
agitator being 
worked by a cam 
on the rear belt 
shaft and the sand 
agitator consisting 
of a worm on a con- 
tinuation of the 
cylinder shaft, this 
being clearly shown 
in Fig. 41a. These * 
mixers are essen- ; 
tially portable, and V, 
can be taken from 
floor to floor as the 
work proceeds. 

Coltrin. Machines of this make are made and the mixer is a friction clutch, which, 
in eight sizes, between which there are besides being generally useful, has one 
various differences of detail, but for the pre- special advantage : should a large stone 
sent purpose the No. 14 machine (Fig. 42) get into the mixer accidentally and bind 




Fig. 39. Gaspary Drum-type Continuous Mixer 



44 



REINFORCED CONCRETE 



between the blades and the vessel, the 
clutch will slip, so avoiding breakage of the 
mixer. The water tank with special pro- 




Fig. 40. Mason Mixer 

portioning arrangement is furnished with all 
machines for use where regular direct pres- 
sure is not available. 

Carey-Latham. In this machine, chain 
and bucket elevators are employed, one for 
the sand and another for the aggregate, to 
feed the mixing cylinder, which is inclined 
towards the discharge end at an angle of 
8 degrees, revolves on friction rollers, and 
has projections on its inner surface to assist 
the mixing. The cement is fed to the 
cylinder by means of a worm. Other types 
of machine bearing this name have been 
used, but the above is the most generally 
useful. 

Gaspary. The funnel-dish mixer made 
by this firm has a number of funnel-shaped 
reservoirs open at the bottom, below them 
being rotating blades. The materials are fed 
into the reservoirs, passed through the 
adjustable openings, dropped upon the pro- 
jecting rims of the blades, and are swept 
down into a mixing trough, in which a screw 
is working and in which the moistening is 
effected. 



of bridging of the sand and other materials,, 
without employing agitators. The appear- 
ance of a typical machine is shown in 
Fig. 4i. The three hoppers with their feed 
mechanism deliver the materials to the 
mixing trough at the rate of from 35 to 
40 discharges per minute for each material. 
The sand and cement drop in the same 
place, and the dry mixing begins imme- 
diately and continues for more than half 
the entire length of the trough. Two water 
sprays are provided, one spraying directly 
into the trough, and the other into the 
aggregate before it is discharged into the 
dry mixture of sand and cement, the object 
being to cause the fine stuff to adhere to 
the aggregate instantly. Each spray has a 
separate regulating valve. Passing through 
the cylinder is a square shaft to which 
numerous paddles are bolted. 

Nims. In this machine, the mixing vessel 
is of cast iron in one piece, shaped some- 
thing like a pair of cubes interpenetrating 
diagonally and revolving on the long axis. 
A belt delivers the materials, which have been 
proportioned in a special form of measurer, 
to the mixer. The mechanism for measuring 
consists of a wheel 15 in. in diameter and 
11 ft. long, revolving under a hopper which 
is 12 ft. long. In the wheel are receptacles 
which receive the materials as they fall from 
the hopper, and, as the wheel continues to 
rotate, deliver them to the belt conveyer 
below. In the hopper are partitions which 
can be moved to vary the proportions. As 
above described, the mixer acts on the con- 
tinuous principle, but by fitting an auto- 



GATE FOR 

AGGREGATE 




Fig. 41. Bolte Mixer 

Kent. A variety of machines are made 
by this firm, the principal being a three- 
hopper machine in which very carefully 
designed arrangements are employed to pro- 
portion the materials accurately. Special 
devices have for their purpose the prevention 



Fig. 41a. Hoppers of Bolte Mixer 

matic gate to the discharge end, it can be 
used as a batch mixer. 

Perfect. As shown by Fig. 45, there are 
in this mixer three distinct hoppers, in con- 
nection with each of these being a revolving 
feed drum with deep flanges at each end, 



CONCRETE: MATERIALS AND MIXING 



and a regulating gate for proportioning the 
material set by a lever moving over a gradu- 
ated quadrant. The feed drums make only 
one or two revolutions a minute, and a 
regulator strikes of? the cement and allows 
a fixed quantity to pass as a layer of uniform 
width and thickness. The stream of cement 
combines with a stream of 
sand, and then falls on a 
stream of aggregate, passing cc 
thence to the mixing cylin- 
der, in which is an adjust- 
able water spray. 

Trump. This type of 
machine requires but little 
power, as gravity to some 
slight extent is utilised. 
The measuring device consists of three 
cylinders for the sand, aggregate, and 



45 

the knife can be altered to vary the pro- 
portion. The principle of the measuring 
device is clearly illustrated by Fig. 46. 




Fig. 43. Coltrin Mixing Blades 

The materials, having been proportioned, 
flow together in the form of one stream 




Fig. 42. Coltrin Mixer 



cement respectively, and from this the 
materials pass to three rotating tables, 




Fig. 44. Kent Mixer 

where they meet fixed knife edges that 
scrape the materials off into a trough 
below in required proportions. The set of 



Fig. 45. Perfect Mixer 

(see Fig. 46), which falls into a cylinder, 
where the materials are moistened by 
water sprayed from a perforated pipe. In 
the cylinder, steel paddles rotate on a shaft 
and complete the mixing. 

TESTING EFFICIENCY OF MIXING 

Of the many methods that have been 
proposed for testing whether concrete has 
been properly mixed, the most practical 
and the one best suited for adoption in 
everyday work is that due to Dr. J. S. Owens, 
and described by him before the Society of 
Engineers. It is based on the principle of 
sampling a heap of concrete at different 
places, and then seeing whether all the 



REINFORCED CONCRETE 



samples contain the same relative propor- 
tions of stone, sand, and cement. Each 
sample is put into a tall glass cylinder 
nearly filled with water, shaken up, and 
allowed to settle. The rate of settlement 
of cement in water is about thirty times 
as slow as that of the sand, which, in turn, 
is slower than that of the larger particles of 
stone ; as a result of the variable rates of 
deposition the stones, sand and cement 
settle in distinct layers, whose depth is pro- 
portional to the amount of the material 
present in the sample. Thus it is possible 
to tell, with a fair degree of accuracy, 
whether each sample was compounded with 
the same proportions of cement, sand, and 
coarse aggregate ; obviously, the test re- 
quires reasonable care in its execution. 



barrows of the usual type, or they may 
have an iron framework supporting a steel 





Fig. 47. Ransome Concrete Cart 



CONVEYING CONCRETE 

It always happens that either the 
raw materials have to be conveyed 
from their storage place to the stage 
or mixer or that the concrete is made 
close to where the materials are stored 
and then has to be carried to the 
place where it is to be used. The 
means of conveyance employed em- 
brace barrows, handcarts, derrick 
hoists, bucket elevators, and special 
hoisting systems, such as the Ransome, 
that have been particularly designed 
for use in connection with concrete 
buildings. 

The barrows may be deep wooden 



Fig. 46. 
" Cut-away " 
View show- 
ing Principle 
of Trump 
Mixer 



box in which the concrete is 
carried, the design being such 
that the load is carried as much 
over the wheel as possible. 
A convenient type of iron 
barrow is arranged in such a 
way that the box is pivoted on 
the axle at each side of the 
wheel, so that it can be tilted 
forward to discharge its load. In 




Fig. 48. Ransome Concrete Skip or Bucket 



Fig. 49. Ground Plan, Top Plan, and Two Elevations of Ransome Tower 




4 8 



REINFORCED CONCRETE 



engineering work, but not often in general 
building work, it is economical to have bar- 
rows or tray dumpers running on fixed rails ; 
the boxes need to be made of substantial steel 
plate strengthened round the top edge to 
avoid bending and buckling, and there must 
be an arrangement for tilting the buckets. 
One of the best forms of hand conveyers is 
the Ransome cart (Fig. 47), which has a work- 
ing capacity of from 5 to 5J cub. ft. of wet 
concrete. The wheels are 42 in. in diameter 
with eighteen staggered spokes, the axle, 
1 J in. in diameter, passing right through the 
cart body ; the tread of the tyre is flat, and 
it is essential to have a good runway. The 
handle is reversible, and the whole cart is 
designed to discharge its contents in a 
second or so. For elevator work, in con- 
junction with a special form of tower 
described in the next paragraph, the cart 
can be fitted with legs. 

Hoisting appliances are frequently neces- 
sary, and their nature will depend some- 
what upon the design of the building. Some- 
times it is convenient for the concrete to 
pass straight from the machine mixer into 
cylindrical steel buckets, which are then 
hoisted by a steam crab or other suitable 
means as required. Another arrangement 
is to instal a bucket elevator of the endless- 
chain type, there being sprockets at the 
lowest and highest points which drive the 
chains to which the buckets are attached ; 
these buckets need to be very strongly made 
and have strengthened rims. Other hoist- 
ing arrangements as used in general building 
work will frequently be found useful, and 
these do not need description here. Space, 
however, must be found for mention of the 
Ransome tower, which has been specially 
designed for concrete work. The bucket 
used is a modification of the one previously 
illustrated, and is shown by Fig. 48. It 
has trunnions which are carried in journals 
at the bottom of a steel frame, which slides 
up and down between wooden guides in a 
well-braced tower (see Fig. 49). The 
bucket is carefully adjusted for balance by 
means of stops until it has a slight tendency 
to tilt forward. When loaded, it is pulled 
up inside the tower with its nosepiece press- 
ing against a front guide, and when the 
bucket arrives at a point in the tower where 
the front guide has been cut away to leave 
a space, as shown at B, Fig. 49, it auto- 
matically tilts forward and discharges its 
contents into a bin from which the concrete 



is drawn off by means of gates into barrows 
or handcarts, or in some other suitable way 
is carried to the work. The hoist tower is 
constructed of light timber, and, built accord- 
ing to Fig. 49, it will accommodate the 
smallest size of Ransome bucket, the 
dimension A being 30 in., whereas in 
the three larger sizes this dimension is 
respectively 37 in., 44 in., and 50| in., the 
other dimensions of the tower varying 
accordingly. It will be realised that the 
arrangement is one of great ingenuity, 
and, at the same time, simplicity. The 
mixer discharges a batch into a bucket at 
the lowest point of the system. The friction 
crab hoist shown to the left of the mixer 
operates a rope or chain, by means of which 
the bucket is raised until it automatically 
tilts forward and discharges. The bucket, 
on descending automatically, rights itself 
and comes to rest on a rubber cushion in 
readiness for the next batch. The bucket 
may take other forms, and may embrace an 
arrangement for dumping through the 
bottom when an attendant raises a handle 
or lever. 

Gravity System. The " gravity sys- 
tem " of placing concrete has lately come to 
the fore. Briefly, it consists in hoisting the 
concrete by means of a skip and pouring it 
into a hopper supported at the top of a 
skeleton tower. From this hopper the con- 
crete passes as required through pipes which 
discharge the concrete directly into the 
forms. This system has been used success- 
fully for all classes of work, including an 
eight-story office building and long bridges, 
in one case the concrete having been con- 
veyed by gravity a distance of 502 ft. 

LAYING AND POURING CONCRETE 

It is desirable to deposit concrete in the 
forms or moulds as gently as possible, and 
in such a way as will allow of thorough 
compacting, a result which will be facilitated 
by working with a spade or punner, until 
the surplus water appears on the surface. 
Gentle punning consolidates the concrete 
and is of the utmost value in producing 
good work. Of course, punning must not 
continue after the cement has begun to set. 

The depth to which the concrete is 
deposited obviously depends entirely upon 
the nature of the job. To say that common 
sense should be employed, and the concrete 
never deposited so thickly that there is 
doubt as to whether the reinforcement is in 



CONCRETE: MATERIALS AND MIXING 



49 



contact with the concrete at every part, is 
better than to specify layers of any par- 
ticular thickness. Much has been said as 
to whether concrete may properly be tipped 
from a height into its final resting-place, 
such a method having been thought at one 
time to be necessary in order to consolidate 
the material. The method is not harmful 
where the vertical distance or " head " is 
only a few feet, but it is not to be recom- 
mended when the distance is any more than 
that, as there is a tendency for the heavier 
ingredients to separate from the bulk of the 
concrete. 

The very greatest care should naturally 
be exercised to prevent any interference 
with the wet concrete after it has been 
deposited and rammed. Any disturbance 
after setting begins is detrimental to the 
strength. 

It is usual to cover up the concrete at 



night, one object of so doing being to prevent 
any violent rain washing some of the cement 
out of the top surface, and another, in winter r 
to prevent frost reaching the work, since it 
appears that frost occurring before the 
cement has set exercises a disintegrating 
action, and influences the setting power and 
ultimate strength unfavourably. 

When depositing concrete in layers, it is 
important to secure a good bond between 
the old and the new material. When there 
has been any length of time between the two 
layers, it is necessary to wet the old work, 
hack it over, and sweep clean. Many 
authorities take the precaution of treating 
the first layer, after hacking and sweeping, 
with cement grout, so as to ensure a strong 
bond. When one layer follows the preceding 
one at a close interval, all that is necessary 
is to see that the surface of the concrete 
already deposited is wet. 



Steel 



Composition of Steel. Steel consists of 
iron containing carbon in the form of iron 
carbide, the proportion of which, and the 
manner in which it is held, largely deter- 
mining the character of the steel. Other 
elements are also present. Steel always 
contains manganese, and also small amounts 
of silicon, sulphur and phosphorus. The 
proportions of the two latter are important, 
and only traces not exceeding 0-04 per cent, 
are permitted in structural steel. The table 
given below indicates the relative composi- 
tion of the various forms of iron in use. 



to the method by which they have been 
produced. They are cast, not forged, into 
shape, and the metal is not malleable in 
the above sense. The bending and twisting 
of wire used in reinforced concrete work 
for tying the bars or making meshed 
wire supports requires that the metal to be 
used shall possess the property of malle- 
ability. 

Tenacity. The resistance to rupture by 
a stretching force is known as tenacity. 
It is determined by subjecting a piece of 
the metal of known sectional area to a 



Silicon 


Carbon 


Sulphur 


Phosphorus 


Graphite 


Combined 


Cast iron (castings)* 
Wrought iron 
Mild steel 


13-5 

tr. 0-1 
tr. 0-3 


13 


0-150-9 02 
tr. 0-25 tr. 0-1 
0-10-5 0-31-0 


00-2 
tr. 0-1 
tr. 0-04+ 


01-5 
tr. 0-2 
tr. 0-04| 


Harder steel 


tr. 0-3 





0-60-8 1 ditto 


ditto 


ditto 


Cutlery steel 








0-51-5 tr. 0-3 


trace 


trace 



Not pig iron or malleable cast iron, 
f For rails this may be 0-06-0-07. 



Alloy steels contain, in addition to carbon, 
other metals which give their name to the 
steel nickel, chromium, tungsten, molyb- 
denum, vanadium, and manganese and 
each of these alloy steels has some special 
character. 

It will be seen that the tough, tenacious, 
and malleable forms of iron are, with the 
exception of alloy steels, those containing 
only small amounts of foreign bodies other 
than carbon, and that as that element 
increases, the hardness and elasticity increase. 
Cutlery and tool steels contain the larger 
amounts of carbon. 

Malleability. No confusion should exist 
as to the term " malleable " applied to iron 
and steel. Metal that can be forged, rolled, 
bent and worked at a red heat, and to a 
less extent in the cold, is malleable. The 
term is often applied without qualification 
to certain kinds of castings which have had 
their brittleness removed by treatment. 
The composition of these varies according 



gradually increasing stress in a testing 
machine till rupture takes place. The force 
applied is indicated by the machine. In 
Great Britain, tenacity is usually stated in 
pounds or tons per square inch of sectional 
area. If a test-piece of 0-7979 in. diameter 
broke when subjected to a load of 30,000 
lb., the tensile strength of the material 
would be 60,000 lb. or 26-78 tons, since the 
bar is half a square inch in sectional area. 
The tensile strength varies with the 
composition, and the treatment of the 
metal. In carbon steels that have been 
heated above 900 C. and allowed to cool 
in the air normalised the tenacity in- 
creases as the percentage of carbon rises, till 
it attains a maximum of 61 tons at about 
1-2 per cent, carbon. Steel that has been 
annealed by heating the metal to redness and 
allowing to cool very slowly has a lower 
tenacity. The maximum of about 36 tons 
is reached with 0-9 per cent, carbon, above 
which it falls away again. The graph 



STEEL 



presented by Fig. 50, and the figures on the 
preceding page are given by Prof. Arnold. 

Increasing the rapidity of cooling by 
quenching from redness may raise the 
tensile strength, but affects also other 
properties. The smaller the amount of 
carbon present, the less is the effect produced 
by varying the rate of cooling. Very soft 
steels, containing little carbon, are not 
greatly affected, as will be observed from 
the diagram just referred to. 

The divergence in the indicated strengths 
between the normalised and annealed speci- 
mens becomes more marked as the MAX. 
percentage of carbon increases. | N T To S N S s 
The quenching effect also follows the P ER so IN. 
same course, and is much greater 60 

with the higher carbon steels. They at 
the same time become much harder and 
more brittle. The hardening effect 50 
becomes sensible above 0-35 per cent., 
and with 045 per cent, is quite distinct. 
With 1 per cent, the metal becomes 
brittle and very hard. 

Under ordinary treatment a percen- 
tage of carbon below 0-8 per cent. 

'^n 

raises the tensile strength, but does 
not make the metal brittle. Steel 
containing from 0-7 to 0-85 of carbon, 
although its ductility has been greatly 20 
reduced, is quite suitable for reinforced 
concrete construction, as it is still suf- 
ficiently ductile to be free from liability 10 
to breaking under ordinary conditions ; 
it is, however, much stiffer and more 
rigid and elastic than metal having o 
a lower carbon content. Do steels with 
higher carbon contents, used in plain 
bars, and not ridged, notched or other- 
wise " deformed," secure a more per- 
manent grip on the concrete than do 
steels with lower carbon contents ? It is a 
moot point. 

Ductility. The power of being drawn 
out or extended is known as " ductility," 
and it is indicated by the extent by 
which the test piece in a tensile test 
elongates before fracture. This elongation 
is expressed as a percentage of the original 
length of the piece. Before testing, two 
centre- punch or other marks are made on 
the piece, at a carefully measured distance 
apart. After fracture the two portions 
are laid together and the distance again 
measured. The increase is calculated in 
percentage terms of the original length. 
In stating elongation, the length of the test- 



piece must be given, since, owing to the 
extensive thinning of the bar at the point 
of fracture, much of the elongation is con- 
fined to that portion in which the fracture 
occurs. This is the case with both short 
and long test-pieces, but on the short pieces 
the local elongation represents a higher per- 
centage of the original length. 

Ductility decreases as the carbon increases, 
as the graph (see Fig. 51 on next page) 
by Prof. Arnold shows. Here, again, the 
effect of rapid cooling is plainly indicated. 
Carefully annealed material shows a mini- 











>A 












/ 


/ 


^ 






/ 


7^ 








/ 


/ 
L 


^*^ 


^ 




/ 

<y ^ 


^ 


/ 






\ 

A 


*T 

A 
























25% '5% -75% \'Ql \'25% \'5% 

CARBON 



Fig. 50. Graph Showing Influence of Carbon on 
Tenacity of Steel : N, normalised specimens, 
and A, annealed specimens 

mum ductility at about 0-89 per cent, carbon, 
after which the ductility rises. With normal- 
ised metal the decrease continues to little more 
than 2 per cent, at 147 per cent, carbon. 

The difference in the behaviour of the 
metal in the two states results from the 
difference in the manner in which the car- 
bide is held. The carbide dissolves and 
diffuses through the metal when heated. 
If slowly cooled it separates completely 
from the iron during the cooling. More 
rapid cooling prevents the separation from 
being completed, while quenching may 
prevent the separation altogether. Thus the 
properties of the metal vary with the rate 
of cooling and the amount of carbide. 



REINFORCED CONCRETE 



Prom the above it will be seen that for 
material that must be subjected to severe 
treatment in forging and use, such as boiler 
plates, rivets, etc., only soft, low-carbon 
steels can be applied. Steel containing 
more carbon would be too hard and not 
sufficiently malleable and ductile, so that 
cracking or fracture might occur. With 
materials which are not subjected to severe 
treatment and where high tensile strength, 
elasticity, and rigidity are more important, 
higher proportions of carbon are allowable. 
Thus, with 0-9 per cent, carbon, a tensile 
strength of 51 tons per square inch and 



ELONGATION 
PER CENT 

ON 2" 



50 



40 



30 



20 



10 







25% 



5% 



75% 



'25% 



Fig. 51. Graph Showing Influence of Carbon on 
Ductility of Steel 



an elongation of 14 per cent, are obtained, 
the latter being somewhat low, but higher 
than in the case of much wrought iron, the 
tensile strength of which is less than that 
of the steel. 

Elasticity. This is the power of the 
steel to recover its original shape, and it 
is influenced by the manner in which the 
carbon is held in the metal. 

Limit of Elasticity. This term is 
applied to the force which will produce a 
permanent set or extension of the metal 
under tensile test ; that is, the point at 
which the metal ceases to be perfectly 
elastic. It is usually about half the total 
strength. The metal will continue to 
stretch on the further application of force, 
but the extension is not then proportional 



to the force applied, and the metal does 
not recover its original dimensions on the 
removal of the stress, but is permanently 
strained. 

Testing Steel. Fig. 52 shows test- 
pieces. They are carefully machined to 
shape, marked by means of a centre -punch 
for determining elongation, and the ends, 
firmly gripped by the jaws of the machine. 
Force is gradually applied to pull the ends 
apart and the piece carefully watched. At 
a certain point, a change in the outer 
skin may be observed. With a lever 
machine the sudden elongation may cause 
the lever to drop. This point is 
the commercial elastic limit. Fur- 
ther application of force produces 
further extension, and ultimately 
a point is reached when the in- 
creasing load causes the piece, 
which up to this point has been ex- 
tending uniformly over its whole 
length, to begin to contract locally, 
and rapidly to diminish in sectional 
area ; in this region the piece ulti- 
mately breaks. The contraction in 
area is generally stated as a percent- 
age of the original area. The frac- 
ture on examination should be 
uniform, free from flaw or defect, 
and silky or finely granular. Kound 
pieces should show cup and cone 
formation if soft. Fracture should 
occur near the middle, and with 
uniform material there should be 
'* 5 * only one region of contraction. Tests 
in which this does not occur or 
which break very near one end are 
not satisfactory. The effective 
length that is, the distance between the 
marks varies from 2 in. to 10 in. ; but 
usually 2-in., 6-in., 8-in., or 10-in. test 
pieces are employed. 

SPECIFICATION FOR STEEL REINFORCE- 
MENT 

The following is a suitable specification 
for steel reinforcement : 

The steel used shall be manufactured 
by the Open Hearth process, and shall 
be of the very best description, free from 
blisters, scales, laminations, and shall be 
of the size and weight specified (a vari- 
ation of 2 per cent, either way is usually 
allowed). 

The chemical analysis shall show per 
cent, carbon, and not more than the fol- 



STEEL 



53 



lowing percentages 
sulphur : 

Phosphorus 
Sulphur 



of phosphorus and 

. 0-06 (maximum) 
. 0-04 (maximum) 



On this assumption a force of 1 Ib. would 
extend or reduce the length of a test-piece 

of the oriinal 



No allowance above these figures must be 

made. The steel shall have an ultimate 

tensile strength of not 

less than 28 to 32 

tons per square inch, 

and an elongation of 

at least 15 per cent. 

on an 8-in. length. No 

welds shall be made in 

the steel under any 

circumstances. When 

forging is necessary 

special attention shall 

be paid to the smith's 

work. No overheated or burnt steel to be 

used. All bending (where possible) of 



CERTIFICATE OF TESTS 

The following is a sample certificate of 




t ~ 



Fig. 52. Test Piece Before and After Stretching 



tests for steel used in reinforced concrete 
construction : 



CERTIFICATE OF TEST 
Messrs. X. 

Tests of Mild Steel to your order. 
For delivery to A. B. 



No. of Sample 


Original size 


Contraction of Area 


Elongation 
on 8 inches 


Ultimate 
stress 


Elastic limit 


^ 


1 


l-i 


|i 


3 4 
5 8 

S 1 

8* 


1*1 


j 


S3 


Hj 

S't 


fi 


'". 


1J Rounds . 
1 X i Flats . 
Ditto . 
1 inch square 


1-12 
1-01 x 0-495 
1-00 x 0-505 
1-OOx 0-99 


0-985 
0-5 
0-505 
0-99 


0-76 Diam. 
0-79 x 0-295 
0-75 X 29 
0-7 X 0-69 


0-454 
0-232 
0-217 
0-482 


54 
53-5 
57-0 
51-3 


10-22 
10-24 
10-16 
10-25 


27-7 
28-0 
27-0 
28-2 


28-6 
14-6 
14-4 

29-8 


29 
29-2 
28-5 
29-9 


18-64 
9-75 
10-10 

20-8 


18-9 
19-5 
19-9 
21 



1 X J Bend tests satisfactory. 

Cold Bend tests satisfactory. 

1 X J Flats cold bend tests satisfactory. 



small metal to be done cold. Larger 
metal to be heated to a dull cherry 
red. The bending force must be applied 
gradually and regularly. Before use, all 
steel must be clean and free from scale and 
rust. No oil or paint must be used on the 
steel. 

Modulus of Elasticity. This is the 
force in pounds that would be required to 
extend a bar to double or compress it to 
half its original length on the assumption 
that it remained perfectly elastic throughout. 
It is frequently called the coefficient of 
elasticity ; thus for steel the modulus is 
about 30,000,000 Ib., and the coefficient 

* s ^ a ^ v constant for open- 



JO 000 000' 
liearth steel with varying carbon contents. 



Fatigue. When subject to varying and 
fluctuating stresses, metals are liable to 
alteration. The tensile strength, ductility, 
and toughness are reduced. In some cases 
this is very pronounced, and the strength 
may fall below the ordinary working load 
and fracture take place. The alteration 
under ordinary conditions takes place slowly 
in steels that are free from injurious con- 
stituents and of uniform texture. The 
elasticity generally depends on the latter 
condition, and metals of high elasticity are 
less liable to fatigue. The elastic limit in 
the case of steel is higher than for wrought 
iron, and its resistance to internal change is 
greater. 

Toughness. This may be defined as 
the resistance to fracture by bending beyond 



54 



REINFORCED CONCRETE 



the elastic limifci It is especially necessary 
in steel that must be bent or twisted. Soft 
malleable steel, of uniform texture and 
character, is toughest ; but very much 
depends on the heat treatment the steel 
has received. 

Hardness. This is difficult to determine 
with accuracy, and for reinforced concrete 
work is only of secondary importance. It 
is dependent on the heat treatment and rate 
of cooling of the metal, and on the percentage 
of carbon present in the steel. Reference 
to the following table, and a consideration 
of the uses of steels of varying composition, 
will illustrate the latter point, and the study 
of the effects of carbon to be dealt with 
later will explain the cause of the increased 
hardness produced by quenching from a 
red heat. 



Steel 


Carbon contents 



Soft malleable metal 
plates and rivets . 



for 



For ordinary wire for ropes 

Hard wire for guide ropes . 

Rails .... 

Rolled sections 

Hard open-hearth 

Tool steel for dies and steel- 
ing purposes (axes and 
plane irons, die temper) . 

Setts, minting dies, smith's 
tools (sett temper) 

Cold chisels, miners' drills, 
large punches (chisel 
temper) 

Circular cutters, taps, 
rimers, large turning 
tools and drills, screwing 
dies (punch temper) 

Turning tools . 

For small tools, sawfiles, etc. 

For razors and special 
purposes 



01 0-3 
0-2 04 Ma. 
0-150-3 
0450-75 
0-3 0-6 
0-220-25 
0-6 0-8 



0-75 

0-825 

1-00 



1-125 

1-25 

1-375 

1-5 



matter included in the steel. Overworking 
the steel may produce lamination. 

Usually with increased diameter a smaller 
demand is made for elongation. For an 
increase of in. above f in. a deduction 
of 1 per cent, is made, for decreases of j\ in. 
below yV a deduction of 2| per cent, from 
the specification. 

Bending Tests. These consist of bend- 
ing the metal as per specification upon itself. 
Rivet steel is tested at full size as rolled. 
It must bend flat on itself through 180 
without fracture on the outside of the 
bend. All material less than f in. is tested 
at full thickness, but above that thickness the 
specimen used is in. thick, and if possible 
1 in. wide. Soft steel must bend over on 
itself through 180 without fracture at 
any part of the bend. Medium steel must 
bend through 180, round a diameter equal 
to the thickness of the metal, so that the 
two limbs are separated by a distance equal 
to the thickness of the test, without showing 
sign of fracture on either side. For works 
tests, the pieces are often doubled closely 
together. Tests after quenching from a 
red heat are somewhat less severe, the 
distance between the limbs being equal to 
1| times the thickness of the metal. 

The importance of a high elastic limit 
will be appreciated when the effect of 
subjecting to tensile strain a smooth round 
bar, one end of which is embedded in con- 
crete, is considered. In such circumstances 
the load may be increased till the elastic 
limit is reached without in any way per- 
manently lessening the grip of the concrete 
on the metal. When the elastic limit is 

TENSILE STRENGTH AND ELASTIC LIMIT 
OF STEEL IN TENSION AND COMPRESSION 
IN POUNDS PER SQUARE INCH.* 



High-speed steels containing tungsten, 
chromium, and other metals to render the 
metal self-hardening contain only 0-5 0-7 
per cent, carbon. 

Referring to the specification already 
given, freedom from blister may be secured 
by using ingots free from blow holes. Scales 
and laminations result from faulty treatment 
in the forging or rolling, or to some foreign 



Percentage 


Ultimate 


Elastic 


Elastic 


of tensile 


limit in 


limit in 


Carbon 


strength 


tension 


compression 


0-14 


62,944 


41,955 


39,536 


0-19 


68,096 


47,084 


43,008 


046 


75,712 


49,056 


48,944 


0-55 


80,416 


46,995 


49,772 


0-66 


89,600 


53,244 


53,648 


0-78 


92,064 


53,312 


53,648 


0-82 


102,816 


57,003 


63,168 


0-87 


104,608 


61,017 


56,000 


0-96 


118,048 


69,216 


71,120 



* Quoted from Twelvetrees' " Concrete-Steel." 



STEEL 



55 



reached, the permanent extension with the 
accompanying thinning of the bar would 
release it as far as contraction extends, and 
this would probably progress along the bar 
by the repetition of the stress. 

Shearing Strength. The value of this 
factor is very largely influenced by the 
structure of the steel developed in rolling 
and the heat treatment. Usually it amounts 
to 70 to 75 per cent, of the ultimate tensile 
strength. 

MODULI OF ELASTICITY OF STEEL WITH 

DIFFERENT PERCENTAGES OF CARBON 
IN 1,000,000-LB. UNITS. 



Carbon 


Tension 


Compression 


Bending 


Mean 


0-19 


30-8 


37-0 


29-1 


32-9 


0-46 


32-0 


32-7 


29-3 


31-8 


0-54 


30-6 


36-1 


28-8 


32-4 


0-66 


32-4 


35-7 


32-1 


33-6 


0-78 


33-5 


32-4 


30-1 


32-4 


0-87 


31-0 


31-5 


30-4 


31-1 


0-96 


30-9 


32-7 


29-2 


31-2 



Resistance to Alternating and Re 
peated Stresses. For all structural pur- 
poses the permanence of the steel is of the 
highest importance. Repeated stresses in- 
duce a brittle condition which is not in- 
dicated by the testing machine. The piece 
breaks as though it were absolutely brittle. 
Tests of brittleness are made by bending 
small prepared test-pieces in opposite direc- 
tions through a small angle by means of a 
suitable machine. The number of bendings 
before fracture occurs varies with the com- 
position and the treatment the steel has 
received. Mr. Stead gives the following 
figures for steel containing 044 per cent, 
carbon : 



Normal 
Overheated 
Reheated . 
Annealed . 
Sorbitic 



1,432,500 
844,950 
2,080,440 
1,971,000 
3,517,200 



The effect of subjecting bars to mechanical 
treatment, and the consequent variation in 
the results obtained, is evidenced in the 
table given below. The bars tested had 
been twisted at one end while cold. 



The rate of alternation has a very con- 
siderable effect. Prof. Arnold has shown 
that the resistance to rupture is inversely 
proportional to the rate of alternation. 
Steel that has become brittle under such 
conditions cannot be restored by annealing, 
neither can it, indeed, by any other heat 
treatment. 



Description 
of Bar 


Original 
size in 
inches 


Orioinal 
area in sq. 
inches 


Elastic stress 
per sy. inch 


Ultimate 
strength -per 
sq. inch 


Ratio of 
Elastic to 
Ultimate 


Contraction 
in area at 
fracture 


Extension 
in 8 inches 


Extension 
in 10 inches 




5-88 twists per 
foot 




















Plain end 

| square 


377 

X 

382 


0-144 


43,000 Ib. 
= 19-2 tons 


58,847 Ib. 
=26-3 tons 


73-1 


71-5 


27-5 


26-2 


Silky 


Twisted end 


do. 


do. 


59,700 Ib. 
=26-7 tons 


86,250 Ib. 
=38-5 tons 


69-2 


59-7 


6-7 


6-0 


do. 


4 '21 twists per 
foot 




















Plain end 
square 


0-5 

X 

0-5 


0-25 


44,000 Ib. 
= 19-6 tons 


62,400 Ib. 
=27-9 tons 


70-5 


67-6 


28-7 


27-6 


do. 


Twisted end 


do. 


do. 


78,400 Ib. 
=35-0 tons 


88,480 Ib. 
=39-5 tons 


88-6 


56-4 


7-2 


6-3 


do. 


3 - 15 twists per 




















foot 




















Plain end 
f bar . 


0-613 

X 

0-615 


0-377 


31,500 Ib. 
= 14-1 tons 


53,475 Ib. 
=23-9 tons 


58-9 


69-8 


35-5 


33-9 


do. 


Twisted end 


do. 


do. 


64,000 Ib. 
=28-6 tons 


73,820 Ib. 
=33-0 tons 


86-7 


63-7 


7-0 


6-0 


do. 



REINFORCED CONCRETE 



Effects of Heat on Physical Pro- 
perties. Sir W. Fairbairn showed that 
the strength of rivet iron and boiler plates 
(best Yorkshire iron) increased as the 
temperature rose to 435 F., rising from 
62,720 Ib. at 60 F. to 86,016 Ib. at 435. 
At 1290 F. the strength was only 35,840 Ib. 
Mild steel does not so increase in strength. 
Styffe showed that between 212 F. and 
392 F. the strength remained practically 
unchanged. 

Other experimenters, including Howard, 
Barnaby, and Martens, have shown that 
between 60 F. and 150 250 there is a 
fall in tenacity, followed by an increase up 
to 450 or 600 F. Beyond this point the 
tenacity falls, and at 1100 to 1200 F. is 
only about half the original. 

Although an actual increase in strength 
may be registered, the ductility is diminished. 
At about 550 to 570 F. the metal shows 
signs of brittleness, and fracture occurs 
suddenly without elongation. Eesistance 
to shock at or above these temperatures 
is therefore probably much diminished. 
This temperature, be it noted, is much below 
redness. 

The minimum length of a bar to be em- 
bedded in concrete in order to ensure its not 
slipping under tension will depend on the 
adhesion between the two substances, the 
elasticity and the area of the metal surface 
embedded. Until the bar stretches the 
adhesion will remain unimpaired. It may, 
however, be affected by unequal expansion 
of the materials, when the composite 
material is subjected to heat and also to 
internal stress of a variable character such 
as vibration and shock. Any advantages 
derived from the shape of the bar is due 
either to the increased area caused by the 
shape given or to the irregularity of form, 
securing a bond when the natural one may 
have been reduced or destroyed. 

The adhesion varies greatly, and is in- 
fluenced by the nature of the cement, the 
amount of water used and other factors. 

Coefficient of Expansion. The co- 
efficient of expansion of steel is 0-000066 
for mild steel, and 0-000069 for hard steel. 

Other Elements Present in Steel. 
Beside carbon, steel also contains small 
amounts of other elements. 

Silicon. Usually only traces of this ele- 
ment are present, being the remains of what 
was present in the pig iron used in making 
the steel, and which was not eliminated 



during the process. It is generally below 
0-1 per cent. In such amounts its effects are 
negligible. Small amounts of silicon in steel 
have the effect of ensuring sounder metal 
freer from blowholes, and it is often added 
for this purpose to the metal before casting. 
In larger amounts, silicon has the effect of 
largely increasing the size of the crystal 
grains, and with 2-5 per cent, the metal 
presents on fracture an appearance more or 
less like spiegel. Further increase produces 
a fracture resembling silicon iron. Turner 
found that up to 0-315 silicon had no effect 
on the tensile strength or ductility. With 
0-5 per cent, an increase of 7 tons in the 
tensile strength and a diminution of 4 per 
cent, on the elongation took place. 

Larger amounts of silicon have the 
effect of raising both the tensile strength 
and the elastic limit of the metal. 
With small proportions of silicon, no loss 
of ductility occurs, but with amounts beyond 
2 per cent, the loss in ductility is very 
marked, and with 4 per cent, the ductility is 
practically nil. The metal remains malleable 
if the carbon content is low, until more than 
6 per cent, of the element is present. Beyond 
this it is brittle. With increased carbon less 
silicon will produce brittleness. Silicon up 
to 2 per cent, has no effect on the hardening 
of steel by quenching in water even from 
a welding heat. More than this renders the 
steel somewhat stiffer, but not harder. 
Silicon steels are being used for tram rails 
and in work where high elasticity is required. 
It has been applied to the making of springs 
for motor-cars and other vehicles For rails 
0-3 per cent, with 0-5 0-6 per cent, carbon 
and 1 per cent, manganese, and for the latter 
0-8 0-9 per cent, silicon and 0-7 per cent, 
carbon and 04 per cent, manganese. With 
the latter alloy, very careful treatment is 
necessary to develop the mechanical pro- 
perties. Carefully annealed, the tensile 
strength is about 50 tons, with an elastic 
limit of 30 tons, and 14 to 18 per cent, 
elongation. After quenching at about 900 
C. and reheating to 500 C. the tensile 
strength is 85 95 tons, elastic limit 63 67 
tons, with an elongation of 5 to 12 per cent. 
The table on p. 57 is quoted from Greenwood 
and Sexton's " Steel," and shows the chief 
mechanical properties of the metal. 

Manganese is a constant constituent of 
mild steel. Iron free from or containing 
little carbon cannot be melted and cast 
without becoming burnt. Manganese is 



STEEL 



57 



Analysis 


Unannealed 


Annealed 


Per cent. 


Tons per sq. in. 


Per cent. 


Tons per sq. in. 


Per cent. 


C. 


Si. T.S. 


E.L. 


E. 


C. 


T.S. 


E.L. 


E. 


C. 


0-14 


0-24 


33 


22 


30 


54 


25 


15 


37 


61 


0-18 


0-73 


34 


25 


29-5 


54-5 


29-5 


19 


34 


52-5 


0-19 


1-6 


37-5 


28 


31 


50 


33 


25 


35 


54-5 


0-20 


2-18 


39-5 


31 


18-5 


28 


34 


25-5 


36-5 


60 


0-20 


2-7 


42-5 


32 


17-5 


24 


32 


24 


6 


6-5 


0-21 


3-4 


47-5 


35 


11 


14 


39 


30 


9 


9 


0-25 


4-2 


49 


45 


0-004 


0-2 


38 


none 


0-64 


0-98 


026 


5-5 


48 


none 


0-3 


0-7 


25 


25 


0-37 


0-5 



added in the form of spiegeleisen or ferro- 
manganese to deoxidise it and thus restore 
malleability. Its tendency is to make the 
metal harder and to raise the tensile strength 
slightly, but to reduce the ductility. It 
may perhaps correct to some extent the 
effect of sulphur in producing red shortness. 
For structural steel the amounts present 
vary from 0-2 to 1 per cent. The latter is 
allowed in rails and is occasionally exceeded. 
Generally the amount is below 0-6 per cent. 

Phosphorus. This element is present in 
steel as the phosphide, containing 15-57 
per cent, of phosphorus ; that is, phosphorus 
produces nearly 6J times its weight of the 
compound, and the effects must be judged 
in this light. Its general effect is to produce 
cold shortness, brittleness, and sensitive- 
ness to shock. It produces a crystalline 
structure. The metal rolls well hot, and 
its tensile strength and elasticity are raised, 
but the ductility is reduced, and may be 
completely destroyed. The effect on the 
ductility and toughness is intensified by 
the presence of carbon. With amounts 
under 0-1 per cent, but little change can 
be observed in the metal under test, but 
its sensitiveness to shock may be very greatly 
increased. Although its behaviour varies, 
its reliability in this respect is destroyed. 
It is necessary therefore to specify a maxi- 
mum. For rails and similar purposes a 
maximum of 0-08 is permissible, and 0-07 is 
usually specified, but 0-04 is often required 
in steels containing 0-4 to 0-6 carbon. 
With higher-carbon steels less phosphorus 
is allowable, and the lower figure should be 
adhered to. For tool steels the phosphorus 
should not exceed 0-02 per cent. 

Sulphur. This element also exercises, 
even in small quantities, a serious influence 
on the properties of steel. It induces red 



shortness when the amounts present are 
not sufficient to produce any effect on the 
tenacity or ductility of the steel in the 
cold. The steel cracks on rolling even when 
the sulphur present is less than 0-1 per cent., 
and the steel is therefore very unreliable, 
since, although externally satisfactory, in- 
ternal cracks and flaws may be produced 
which, under the conditions of application 
vibration, variable temperature, etc. 
may ultimately weaken the steel below its 
working load and produce fracture. It is 
most likely to produce fatigue, and its 
presence destroys the welding power of the 
steel. Not more than 0-05 per cent, should 
be allowed in any circumstances. 

Carbon. As already noted most of the 
changes in the mechanical characters of 
steel are produced by the carbon present. 
Not only does an increase in the element 
produce a change in properties, but 
different properties may be presented 
by steels containing the same amount, if 
they have received different heat treatment. 
Steel heated to redness and quenched 
is made harder to an extent depending 
on the amount of carbon present. 
The same steel, if reheated and allowed to 
cool slowly, is softened. In the softened 
state it is harder than iron free from carbon. 
The tensile strength is raised, but the 
ductility is lowered. The amount is not, 
however, in proportion to the increase of 
strength. This renders carbon the best 
hardening agent, as it produces the greatest 
increase in tenacity with the least diminution 
of ductility. In hardening, the change 
occurs at a definite temperature or at least 
over a very limited range of temperature. 
If the steel cools below this temperature 
before quenching, it is not hardened, nor is 
it softened unless this temperature is reached. 



REINFORCED CONCRETE 



Below this temperature it is immaterial 
whether the cooling is rapid or slow. The 
temperature is known as the " critical 
temperature " of hardening, and also as 




Fig. 53. Photo-micrograph of Hard Steel 
(3 per cent, carbon) Cooled Slowly 

the " recalescence point." Steel of suitable 
carbon content and volume actually becomes 
visibly hotter as it cools from a red heat 
when the critical temperature is reached. 
Hence the term " recalescence." It marks 
the temperature at which the carbon 
contents of the steel change their condition, 
this coinciding with the hardening effect. 

Carbon exists in steel in chemical combina- 
tion with iron. The carbide that has been 
separated has the composition Fe 3 C, and is 
insoluble in iron below the recalescence 
point, but soluble above it. If quenched 
from a higher temperature its separation is 
prevented, and it remains in solid solution, 
in which condition it produces its maximum 
effect. The condition it assumes after 
or during separation may seriously affect 
the properties. A microscopic examination 
of polished and etched steel shows that 
when cooled in air the carbide has collected 
into small areas, surrounded by uncar- 
burised iron. The size of these areas in- 
creases with the carbon contents, and with 
0-9 per cent, carbon the whole surface is 
uniform. The grey areas are seen under a 
high magnification to consist of alternate 
plates of light and dark material. One of the 



components is the carbide Fe 3 C (cementite), 
and the other the iron (ferrite) from which it 
has separated. If cooled very slowly the 
structure is coarsely laminated. Cooled 
more quickly it is finer, and the laminated 
structure may be very fine or even granular. 
The latter condition, which is stronger and 
more elastic, is the sorbitic structure. The 
laminated material under inclined illumina- 
tion often shows a play of colour and was 
named by its discoverer, Dr. Sorby, pearlite. 
Heated beyond the recalescence point the 
cementite first dissolves in the iron (ferrite) 
of the pearlite, and then diffuses through the 
mass. On quenching, its condition remains 
what it was in the heated state, and the 
hardening effect is produced. 

If cooled slowly, steels containing more 
than 0-89 carbon first separate the surplus 
cementite, and this continues till the recales- 
cence point is reached. The structure is 
shown at Fig. 53. If less than 0-89 per 
cent, carbon is present, the excess of iron 
(ferrite) first separates, and the carbide 
concentrates into what ultimately become 
the pearlite areas (see Fig. 54), final separa- 
tion and lamination being produced at the 
recalescence point. The more complete 
the separation, the softer the material will 
become, and the less will be the effect of the 




Fig. 54. Photo-micrograph of Steel (Dark 
Areas show Pearlite) 

carbon in raising the tensile strength "and 
elasticity. With largely laminated pearlite 
the ductility is not increased, as must always 
be the effect of producing planes that may 
become planes of cleavage or separation. 
Obviously the qualities of quenched steel 



STEEL 



59 



will depend on the carbon (carbide) present 
in solid solution, and at the recalescence 
point it is only capable of retaining carbide 
equal to 0-89 per cent, carbon. Above 
that temperature it will retain more in 
solution, but this, as shown, separates as 
the temperature falls towards the critical 
point. The solid solution of cementite in 
quenched steel is known as martensite. It 
is harder, stronger, more elastic, and less 
ductile in proportion to the carbon it 
contains. For structural purposes the 
modification of the cooling rate to prevent 
lamination of the pearlite by somewhat 
hastening the cooling, so as to produce the 
sorbitic structure, is sometimes carried out. 




Fig. 55. Photo-micrograph of Laminated 
Pearlite in Steel after Prolonged 
Annealing 

Prolonged annealing and slow cooling will 
produce the coarsest lamination and the 
weakest metal. 

Normal air-cooled metal holds a position 
intermediate between the two. 

With regard to the photo-micrographs 
here shown, Fig. 53 shows hard steel (3 per 
cent, carbon) cooled slowly; the borders 
of cementite (Fe 3 C) surround the pearlite 
areas. Fig. 54 is a low magnification in 
which the dark areas represent pearlite. 
Fig. 55 (a very high magnification) shows 
laminated pearlite in steel after prolonged 
annealing. 

In carbon steels the recalescence tempera- 
ture varies from 655 C. to 680 C. and coin- 
cides with the temperature at which the 
carbide areas assume the pearlitic structure. 
Visible redness is about 580 C. In hardened 
steels the metal is in a condition of internal 
stress, due to the retention of the carbide 
in a condition that is not normal in the cold. 



The raising of the temperature in tempering 
operations gives a certain amount of mole- 
cular liberty, and to some extent the marten- 
sitic condition is destroyed, the tendency 
being for the carbide to assume the normal 
pearlitic condition. Softening takes place 
to some extent, and the brittleness is re- 
duced. The nearer the temperature ap- 
proaches the critical point and the greater 
the latitude given, the greater is the change. 
At the critical point it is completed. 

With mild steels there are other tempera- 
tures at which the rate of cooling is retarded. 
The highest point is at 880 C. Another 
occurs at 735, and the third about 680. 
The latter coincides with the critical point 
already noticed. In steels free or nearly free 
from carbon this is insignificant. As the 
carbon content increases, the upper retarda- 
tions diminish and finally disappear, and 
the whole retardation is concentrated at 
the lower point. The higher critical points 
are ascribed to molecular changes in the 
iron itself. These cannot, however, be 
made permanent by quenching, and in the 
absence of carbon their effect is not notice- 
able in the metal, whether cooled rapidly or 
slowly. 

In alloy steels such as tungsten, chrome, 
chrome - tungsten, molybdenum, nickel, 
manganese, etc., the added metals lower 
the recalescence point and bring it below 
ordinary temperature. Such metals in 
consequence retain their hardness and are 
known as self -hardening or air-harden- 
ing steels. They can be heated without 
becoming softened, and hence cutting 
speeds which develop so much heat as to 
destroy the temper of ordinary steel tools 
can be employed without risk. Hence 
the term rapid-cutting, high-duty, or high- 
speed steels. 

METHODS OF MANUFACTURE 

Blister or cementation steel is manu- 
factured by heating bars of nearly pure 
iron embedded in carbon for a long period. 
Such metal is covered with blisters, hence the 
name. It is usually converted into shear 
steel by welding together a number of such 
bars to secure greater uniformity. 

Cast crucible steel is made by melting 
blister steel in crucibles and casting into 
ingots. Different tempers are secured by 
additions of pure soft iron, carbon, and other 
ingredients to obtain the desired product. 
The principal use of such steel is for cutting 



REINFORCED CONCRETE 



instruments. The process is costly and the 
output limited. 

Bessemer steel is made by blowing air 
through molten pig iron contained in a 
suitable vessel. The metal is introduced in 
the molten state. The carbon, manganese, 
and silicon in the metal are removed by 
oxidation, and sufficient carbon is introduced 
by adding the necessary amount of a 
manganiferous pig iron spiegeleisen to the 
metal. The operation is very rapid, a 
charge of several tons being converted in 
from 16 to 20 minutes. The metal is cast into 
ingots and rolled. In the basic bessenier 
process, the lining of the converter is made 
of basic material calcined dolomite and 
this permits of the removal of the phosphorus. 
The operation is somewhat longer. In both 
of the processes, exact control of the carbon 
contents is difficult. Owing to the rapidity 
with which the operation must be con- 
ducted, there is no time for its determina- 
tion, and it is impossible to hold the metal 
in a sufficiently fluid state for the necessary 
length of time to enable this to be done, as 
all the heat required is generated in the 
vessel by the oxidation going on during the 
passage of the air through the metal. 



Open hearth steel usually specified for 
reinforced concrete work is made from 
pig iron and steel scrap in a gas-fired furnace 
of special design, known as a regenerative 
or open hearth gas-fired furnace. In this 
furnace the materials are melted, and the 
carbon and other elements removed by the 
action of air and by adding iron oxide to the 
charge. The heat required to melt and 
keep the metal molten is supplied by the 
burning gas. The time occupied extends 
over some hours, dependent on the weight 
of the charge and other conditions. Facilities 
are thus afforded for the careful control of 
the contents of carbon and other elements, 
and metal of a definite composition can be 
obtained. In the basic open hearth, a basic 
lining is used, and, as in the basic bessemer 
process, pig-iron containing phosporus may 
be used, as that element is eliminated during 
the process. 

Basic steel is the term applied to steel made 
in converters or furnaces lined with basic 
materials, such as calcined dolomite, magne- 
site, and chromite. 

Acid steel is steel made in converters or 
furnaces lined with siliceous materials, such 
as silica sand and bricks, or ganister. 



Stress Simply Explained 



EXPERIENCE shows that, in a large pro- 
portion of cases, the failure of the practical 
man, and even of the student, to grasp the 
essential ideas underlying the theory of 
construction in reinforced concrete is a 
direct result of his ignorance of the simple 
mechanics of construction, particularly his 
lack of acquaintance with " stress " a term 
meaning the effect produced on a beam by 
loading it. The present intention, therefore, 
is first to deal with the fundamental principles 
that must be understood by anyone desirous 
of successfully studying the constructional 
design of buildings, and then, in the suc- 
ceeding chapter, to show how those princi- 
ples are applied in reinforced concrete con- 
struction. 

It must here be emphasised that it is 
absolutely essential that the reasoning upon 
which the various formulae are based should 
be understood if these formulae are to be 
used intelligently. Students frequently fail 
to grasp the theory of design owing to the 
fact that they do not start with a sufficient 
knowledge of the principles of force and 
resistance. In many instances the very 
expressions that are used are imperfectly 
understood, and the students are endeavour- 
ing to obtain the value for something of 
which they do not know the meaning, and 
often when they have found a result they 
do not realise why it Was necessary to find 
it. To the general reader this may appear 
to be an exaggeration of the state of affairs, 
but we are merely expressing views that 
have been gathered during several years of 
tuition to building students of all kinds. 

FORMULA 

There are two kinds of formulae which are 
met with, namely " rational " and " em- 
pirical." 

" Rational " applies to formulas that have 
been deduced by reasoning alone, such 
reasoning being based upon definite axioms 
known to be correct. It is not sufficient, 
therefore, for the student to know a rational 
formula ; he should also know and under- 
stand the construction or reasoning through 
which the formula was evolved. 



" Empirical " formulae are based upon 
actual experiments through which certain 
values have been obtained, according to the 
material, load, and disposition under the 
test ; and, as such, their application is 
usually limited, and not possible for cases 
which do not agree with the conditions of 
the test. 

Again, it will be necessary to use certain 
factors or constants derived by the person 
responsible for the test, and these constants 
must be accepted as being correct, and 
become merely a matter of memory and not 
of reasoning. There are some cases, how- 
ever, where pure theory is liable to give 
slightly inaccurate results, owing to the 
peculiar behaviour of some materials under 
a varying stress, and any inconsistency of this 
kind would result in the theory not agreeing 
with the practice. An excellent instance of 
the difference between theory and practice 
is that explained in connection with columns 
and struts on a subsequent page. Wherever 
possible, rational formulae are to be pre- 
ferred, and it is with these that the following 
notes will deal in the first instance. 

PRINCIPLE OF MOMENTS 

By far the most important of all the points 
to be grasped is that of the " principle of 
moments," and the definition of this princi- 
ple is often given in flowing language and 
repeated by the student without a proper 
understanding. 

A " moment " really means an amount, 
and is a distinctive term, inasmuch as it 
means not only the actual amount of a force 
in pounds, hundredweights, or tons, but the 
actual value of a force to cause stress at 
any point, this value being dependent on its 
amount, direction, and disposition in relation 
to other forces or constructional members. 
Thus, " taking moments round a certain 
point " really means " taking values or 
amounts acting at that point." 

Now, the " principle of moments " is that 
when several forces are acting at a point 
and equilibrium is produced, then all those 
forces that would tend to cause movement 
in one direction are exactly equalled by all 



61 



62 



REINFORCED CONCRETE 



those forces that would tend to cause move- 
ment in another direction. 

This principle may seem somewhat obvious 
to many readers, and they may not realise 
its value ; but upon consideration it will 
be seen that it is an axiom which is frequently 
used. For example, in the case of a beam 
that carries a load at any point, it is obvious 
that the load is a force tending to cause 
movement in a downward direction, and if 
equilibrium is maintained, then there must 
be some force or forces at work which are 
causing or exerting an upward tendency ; 
this force is supplied by the reaction or 
reactions, which together must equal the 
load. 

LEVERS 

Before investigating the method of ascer- 
taining the value of the reactions with any 
given load, it will be advisable to give a 
few notes on the three orders of the levers. 
If these levers are thoroughly understood, 
matters will be greatly simplified, as most 
problems, both in ordinary construction and 
in reinforced concrete work, can be practically 
reduced to a question of leverage. A lever 
can be denned as a member which turns on 
a point known as the fulcrum, and it is the 
relative positions of the forces acting on the 
lever, and the fulcrum, which give us the 
order of the lever. 

"First Order." A common example of 
a lever is that illustrated in Fig. 56, which 
shows a crowbar being utilised to lift up a 
weight, such as a block of stone. It will be 
seen that the lever is tending to turn on 
the fulcrum under the action of the two 
forces which oppose one another through the 
medium of the lever ; this is an example of 
a lever of the first order, as the fulcrum is 
between the weight and the power. Now 
the value of the power and the weight to 
cause the turning of the lever will depend 
not only upon their actual amount in pounds 
or hundredweights, but also upon their dis- 
tances from the fulcrum. In other words, 
the " moment " of the power at the fulcrum 
is equal to its amount in pounds multiplied 
by its distance to the fulcrum, the latter 
being known as the long arm of the lever ; 
this is, therefore, 20 Ib. x 30 in. = 600 in.-lb. 
The weight is opposing the power, and if 
equilibrium is produced, then the " moment " 
of the weight at the fulcrum must also equal 
600 in.-lb., and this moment is found by 
multiplying the amount of the weight by its 



distance to the fulcrum. Therefore weight 
x 6 in. = 600 in.-lb. 

Weight = f ~ = 100 Ib. 
To produce equilibrium, then, we must have 



20 LB. 




Fig. 56. Lever of First Order 

a weight of 100 Ib. and a power of 20 Ib. 
if the arms of the lever are 6 in. and 30 in. 
respectively. Any fraction over 20 Ib; in 
power would raise the weight of 100 Ib., and 
thus there is a mechanical advantage or a 
gain in the value of the power by the intro- 
duction of the lever. A rule can now be 
deduced which will apply to all levers and 
enable us to ascertain the weight or the 
power required to produce equilibrium if 
the value of one force is known, as follows : 
The power multiplied by its distance to the 
fulcrum is equal to the weight multiplied 
by its distance to the fulcrum. 

" Second Order." In the first order it 
was seen that the fulcrum was situated be- 
tween the power and the weight, while in 
Fig. 57 an illustration of the second order is 
given, where it will be seen that the weight 



POWER 




Rf5l5JANCE. 
OK. WEIGHT 



Fig. 57. Lever of Second Order 



is situated between the fulcrum and the 
power. 

"Third Order." In this (Fig. 58) the 
power is between the weight and the fulcrum. 
The power required will always be in excess 
of the weight, which is not the case with 
the other orders. The student is advised to 
work out examples in each order by the rule 



STRESS SIMPLY EXPLAINED 



given above, in order to familiarise himself 
with the principle of taking moments round 
the fulcrum. 

REACTIONS 

A reaction, as its name implies, is a force 



POWER 



FULCRUM 




RL5I6TANCE 
OE. WE.IOHJ 



Fig. 58. Lever of Third Order 

or resistance which is the outcome of an 
action or force. It is obvious that, when 
any force or action is .introduced, and 
equilibrium is still maintained, there must 
be a reaction or reactions which are equal 
to the initial action, but opposite in tendency. 
No difficulty should present itself in ascer- 
taining the values of these reactions, but the 
student should fully understand the method 
of calculation under any condition of load- 
ing before seeking to understand the theory 
of bending moments. 

The reactions in the case of a beam can 
be denned as the passive resistance at the 
supports offered by the strength of the 
materials at these points to resist crushing. 



/ "! 

' A 

rt-t. 



-f- CCNTBE OP GRAVITY 
/ Op WALL 



CANJILCVER 



-18'- 



V 



Fig. 59. Diagram showing how a Cantilever 
Tends to Move 



The resistance or reaction at each support 
Tvill depend on the amount of the loading 
and its disposition on the beam. 

Cantilever Reactions. In the case of 
a cantilever, as there is only one support, 
the whole of the reaction must be supplied 
by this ; at the same time there is a tendency 
ior the cantilever to overturn, due to the 



leverage of the weight, and lift up the work 
above the tailing-down portion. To prevent 
this overturning movement from occurring, 
a downward reaction or resistance will be 
required at the support, and its value must 
be equal to or greater than the moment of 
the load. This resistance will be supplied 
by the weight of the brickwork or stonework 
over the tail end of the cantilever ; or, if 
this is insufficient, anchor bolts may be 
carried down into the lower part of the wall. 
In Tig. 59 the tendency to move is shown 
diagrammatically by the dotted lines, and 
the problem will be quite simple if the 
arrangement is considered as that of a lever 
of the first order. The overturning moment 
at the point F, which is the fulcrum, will be 
equal to the weight multiplied by its dis- 
tance to this point, which equals 5 tons x 



\ A / 




Fig. 60. Triangular Portion of Wall Lifted by 
Movement of Cantilever 

12 ft. = 60 ft.-tons. The weight of the wall 
above is distributed over the length of 
18 in., and for the purpose of calculating its 
moment at F, it may be considered as acting 
at its centre of gravity, namely, 9 in. from 
the fulcrum. The weight of the wall acting 
at A multiplied by 9 in. or f ft., must there- 
fore equal 60 ft.-tons = expressed as an 
equation : 

A : 

A : 



| ft. 

| ft. 

| A 

A 

A 



= W x I 

=5 tons x 12 ft. 

= 60 ft.-tons 

= 60 ft.-tons -*- | ft. 
= 80 tons 



The weight of the wall over the end of the 
cantilever must therefore exceed 80 tons if 
the latter is to be safe against overturning ; 
or some other form of anchorage must be 
provided which will give an equivalent of 
60 ft.-tons at F. Owing to the bonding of the 



6 4 



REINFORCED CONCRETE 



material of which the wall is composed it 
would be necessary to lift all the work inside 
the dotted lines shown in Fig. 60, and, of 
course, this amount would be considered in 
the calculations. 

Beam Reactions. Certain variations 



load) is situated between the resistance and 
the fulcrum. Therefore : 





W- 

^ 


t 










A 




W 

r 8 




if'f\" 





Fig. 61. Beam Loaded Eccentrically 

occur in the case of beams, as there are two 
reactions to consider, and these are not 
necessarily equal to one another. When the 
beam is uniformly loaded, whether the loads 
are concentrated or distributed, the reaction 
at each abutment will be equal, and their 
combined total must be equal to the sum 
of the loads on the beam. If the loading is 
not uniform, then the reactions will vary, 
and it is these cases that will be dealt with 
here. 

Let a beam be taken with a span of 15 ft. 
loaded with a concentrated weight of 5 tons, 
situated at a point 6 ft. from one support, 
as shown in Fig. 61. A greater proportion 
of the load will be carried by the abutment 
A than by the abutment B, as the weight is 
nearer the former, and the reaction will 
require to be equal to the amount of the load 




Fig. 62. Diagram showing how Eccentrically 
Loaded Beam Tends to Rotate 

carried. The actual amount may be calcu- 
lated by again referring to the principle of 
the levers. In Fig. 62 it will be seen that 
the load has a tendency to push the beam 
downward and exert a pressure on abutment 
A by rotating on the abutment B at point F. 
This is an example of a lever of the third 
order, where the power (supplied by the 



W x 9 ft. 

5 tons x 9 ft. 



= resistance at A x 15 ft. 
= R 1 x 15 ft, 
45 ft. -tons 

15 ft. 
R 1 = 3 tons 

The reaction at A, therefore, is 3 tons, and, 
if the sum of the reactions is equal to the 
load, the reaction at B = 5 tons 3 tons = 
2 tons. This can be proved by considering 
the beam as rotating on abutment A to cause 
a pressure on abutment B, as indicated in 
Fig. 63. Then : 

W x 6 ft. = resistance at B x 15 ft. 
5 tons x 6 ft. = R 2 x 15 ft. 
30 ft.-tons 



R 2 



15 ft. 

2 tons, 

which agrees with the amount above stated. 




RESISTANCE' 
Fig. 63. Beam Rotating on the other Abutment 

A rule can now be deduced as follows : 
The reaction at an abutment is equal to 
the load multiplied by its distance from 
the opposite abutment, divided by the whole 
span. 

If more than one load is carried, then the 
reaction is found by multiplying each load 
by its respective distance from the abutment, 
adding the amounts thus found, and divid- 
ing the total by the whole span ; this will 
be made clear by an example. The example 
in Fig. 64 shows a beam which carries three 
different concentrated loads ; then : 

R 1 = [ (3 tons x 15 ft.) + (4 tons x 10 ft.) 

+ (5 tons x 6 ft.) ] -f- 18 ft. 
45 +40+30 

R = To 



18 



tons 



R 2 = 

(Stons x 3ft.) +(4tons x 8ft.) +(5tons x 12ft.) 
~ 



STRESS SIMPLY EXPLAINED 



9+32+60 



101 



18 
= R 2 = 5^4 tons 



R 1 + R 2 must equal total load 6 T 7 tons + 
5 {* tons = 12 tons total load. 

Any number of loads may be dealt with 
in this manner, and if a distributed load over 
part of the length only has to be considered, 
it can be taken as acting at its centre 
of gravity for the purpose of calculating 
the reaction, while if it is distributed 
uniformly over the whole length it is ob- 
vious that one half will be carried by each 
support. 

The weight of the beam itself has purposely 
been neglected with the object of simplifying 
the explanation, but when it is required to 
allow for this it should be dealt with as a 
distributed load as explained above. 

Effective Span. The beam diagrams in 



4TOI> S70N6 





\ 


i 


r i 


















^ 


, 


. , 




" 


////> 


A \ 




IQ' n f 




l & 




i t 


r2 



Fig. 64. Beam Carrying Three Concentrated 
Loads 

this and the following chapter do not show 
any bearing on the support. What is 
actually illustrated is the effective span, this 
being the distance between the centres of 
the bearing surface, where the pressure is 
theoretically considered as acting. Fig. 65 
shows how the portion of the support inside 
the centre of pressure and the portion of 
the beam outside the same point are neg- 
lected, these portions being hatched. The 
area of the bearing surface will be proportion- 
ate to the amount of the reaction, and will 
vary with the material of which the support 
is composed ; this, of course, is merely a 
matter of dividing the total load on the 
support by the safe load per unit on the 
material, when the required area will be 
given. 

BENDING MOMENTS 

The term " bending " needs no explana- 
tion ; and the term " moment " has already 
been explained ; it will therefore be under- 
5 



stood that the expression " bending 
moment " refers to the amount or value 
of a force or forces to cause a tendency to 
bend. This value will depend on the 
actual amount of the force and on its 
disposition. 



NTEE.OF 5EARIN6 




Fig. 65. Diagram Illustrating Clear and 
Effective Spans 

Bending Moment at Centre of Beam. 

The bending moment (B M) in a beam or 
cantilever is caused by the load and the 
reaction opposing one another ; and, as the 
beam or cantilever is the agent through 
which this opposition is allowed to act, it is 
called upon to resist a certain amount of 
stress due to the bending tendency. 

Assume a beam to carry a load of 10 tons 
in the centre of its length, as in Fig. 66; 
The load will set into force an action which 
can be expressed as a downward action, due 
to the force of gravity ; in order to produce 
equilibrium, this must be counteracted by 
a resistance at the abutments which is equal 
in amount and opposite in direction. There- 
fore, the reactions can be expressed as 
upward forces, which are set into action by 
the passive resistances of the abutments. 
It will be seen, then, that these actions (the 
load and the reactions) exert a bending 
tendency in the beam through which they 
act as shown (in an exaggerated manner) by 





i 




1 




f' 


; 


J 


| 




B' R 2 





Fig. 66. Centrally-loaded Beam 

Fig. 67, in which the beam has a tendency 
to turn on the point F. 

Now, when two parallel forces are acting 
in opposite directions and opposing one 
another, they are said to form a couple, and 
their greatest value to cause stress is the 
amount of one force multiplied by the 



66 



REINFORCED CONCRETE 



distance between them. In the instance just 

W 

given, the greatest value is -^ multiplied by 

I 

o, where W equals the total weight and I 

equals the total span. It is obvious that 




x - which 



Fig. 67. Bending Tendency on Beam 

half the weight is taken by each abutment, 
and therefore the opposition from each end 
of the beam can only be due to this amount. 
This is explained by Fig. 67. Hence the 
formula for finding the greatest bending 
moment in the case of a beam with a central 

W 

concentrated load is B M = -~ 

WZ 
equals B M = -r~> 

W I 

This formula -j- expresses the greatest 

bending moment in the case of a supported 
beam carrying a concentrated central load, 
and this greatest bending moment will be 
found to occur at the centre of the span, 
where moments have just been taken. 

Bending Moment at any Point in 
Beam. Now consider the method of ascer- 
taining the bending tendency at any point 
other than the centre. By taking any 
particular example and calculating the B.M. 
at various points, it will be seen how the 
variation becomes uniform and diminishes 
from the centre of the beam to the abut- 
ment, where it becomes nil. This uniform 
variation will only be found to occur in the 
case of beams and cantilevers carrying con- 
centrated loads, and it will be shown later 
how the variation occurs in the case of dis- 
tributed loads. Assume a beam to carry a 
concentrated load of 10 tons at the centre 
of the span which is 10 ft., as before, then, 

Wl 



according to the formula B M = 



the 



is due to the reaction multiplied by its dis- 
tance to the point at which the moments are 
taken ; and on applying this principle it will 
be found quite easy to ascertain the B.M. 
at any point. Assume the same beam with 
the concentrated load of 10 tons at the 
centre, and find the B.M. at a point 3 ft. 
from one support (see Fig. 68). Now, the 
reaction at each support will be equal to 
5 tons, therefore, it is only necessary to 
multiply the reaction at A by its distance to 
the point at which the B.M. is required, 
namely, 3 ft. Hence, the B.M. = 5 tons 
x 3 ft. = 15 ft.-tons. 

Again, the B.M. at a point 2 ft. from the 
support = 5 tons x 2 ft. = 10 ft.-tons, thus 
showing how the B.M. diminishes towards 
the abutment, and as the reaction is a con- 
stant figure in the calculations for the B.M. 
at any point, and the only factor that varies 
is the distance of the point from the support, 
it can be said that the B.M. at any point 
varies directly as its distance from the support. 

If the B.M. at the centre is set up to scale 
over the beam, as shown in Fig. 69, and the 
span of the beam itself is drawn to scale, 
then the B.M. at all points will be expressed 
graphically by this diagram, and by scaling 
the vertical line at any intermediate point 
the B.M. at that point can be ascertained. 

This explanation has now deduced the 
following rule : 

The bending moment at any point in a 
beam with a concentrated load equals the 
reaction at the abutment multiplied by its 
distance to that point. 









r~ 




*! 

i 

B 1 . 


It!? 




f. T ' s* " 


/ 
\ 

5 TO' 


5"-0 > 

ir>' 


n " 


i i 

5 TOto e*= 



. ' 10 x 10 

bending moment = j = 25 ft.-tons. 

As already explained, the bending tendency 



Fig. 68. Finding Bending Moment of 
Beam 

Weight between Reaction and Point 
of Calculation. If the weight is situated 
between the reaction selected and the point 
round which moments are taken, then allow- 
ance must be made for the moment of 
the weight at this point. 

This can be explained in the following 
manner : Again assume the same condition 
of loading and span and also that the B.M. 



STRESS SIMPLY EXPLAINED 



67 



at a point 3 ft. from abutment B is required 
to be calculated from abutment A (see Fig. 
70). It will be seen that W is situated 
between the abutment selected and the 
point at which the B.M. is to be found. 
Then W is situated on the arm of the lever 



TO6CALE 




Fig. 69. Bending Moment Set up to 

Scale 



with two loads concentrated at different 
points, and we will assume that it is neces- 
sary to ascertain the B.M. at the centre of 
the beam. Then it will be necessary, which- 
ever abutment is selected as the one to work 
from, to calculate with a Weight between the 





f 
. r'-o' 1 - 


(OTOtli ./ 


UPWARD LEVC.RAGE- 
op ts 1 


f , 














m 


f //I ' n" 


*-2'o;-l 


S'O'^-, 


LL 


V 

i 


1 
5 TOS5 DOWNWARD LtVtR 

o^ w 



Fig. 71. Lever Arms of Reaction and 
Load 



working from A, and the downward effect 
of W must be subtracted from the upward 
effect of R 1 at A at the point where the 
B.M. is to be found. 

Then BMatF R 1 x upward leverage 
- W x downward leverage, as shown in 
Fig. 71. 

From this equation it will be seen that 
the B.M. at F actually equals (5 tons x 7 ft.) 
-(10 tons x 2ft.) = 35 ft.-tons -20 ft.-tons. 
= 15 ft. -tons the same result as that 
obtained in the previous calculation for the 
B.M. at a point 3 ft. from one support with 
similar load and span. 

The principle previously explained must 
be thoroughly understood, because it will 
be seen that in the case of a beam which 



3-0'- 



S'C" 



57010 



Fig. 70. Finding Bending Moment of 
Beam 

carries two or more concentrated loads at 
different points it will be quite impossible 
to determine the B.M. at any point between 
two of the loads without considering the load 
which is situated on the arm of the lever. 
Take an actual example showing these con- 
ditions. 

The diagram (Fig. 72) illustrates a beam 



reaction and the point at which moments 
are to be taken. 

Working from abutment A, then 
2 x 12 + 4 x 6 
~ 



Ri = 33 tons. 
B M at centre = R 1 x 
- W 1 x 



distance to centre 
its distance to 

centre. 

B M = 3f- tons x 7 ft. - 2 tons x 5 ft. 
B M = 24 ft.-tons - 10 ft.-tons. 
B M = 14 ft.-tons. 

The following rule can now be deduced : 
The bending moment at any point in a beam 
which carries one or more concentrated 
loads can be found by multiplying either 



W - 2 T&N6 



Z - 4 TON5 





, 




^ 


2.'o"-4 6'-o | 6 -o * 

1 

K' n' > 


'^ 


t 



Fig. 72.- 



-Beam with Two Loads Concentrated 
at Different Points 



reaction by its distance to the point at which 
the B.M. is to be found, but if any weights 
situated between the reaction selected 



are 



and the point at which the moment is to be 
found, then a deduction must be made equal 
to the sum of the weights multiplied by 
their respective distances to this point. 
This rule can be applied to any number 



68 



REINFORCED CONCRETE 



of loads, and once this principle is thoroughly 
understood no difficulty will be found with 
a beam carrying any number of concentrated 
loads, however the latter may be situated. 
Bending Moment in Cantilever. The 
mathematical method of determining the 



W 



Fig. 73. Cantilever with Concentrated Load 
at Outer End 

B.M. with cantilevers carrying concentrated 
loads will now be considered. 

Owing to the fact that only one support 
has to be dealt with, the calculations are 
much simplified, as it will be obvious that 
the reaction is always equal to the load ; 
and no method is required to ascertain this 
as in the case of a beam. 

The most simple case will be found to be 
that of a cantilever that carries a single 
concentrated load at its extreme outer end, 
as in Fig. 73. The greatest bending tendency 
due to this weight will be found to occur at 
the support. This bending tendency is due 
to the weight pressing down on the cantilever, 
and tending to cause failure by turning on 
the point F as shown in Figs. 74 and 75. 

Now if the moments round this point F 




Fig. 74. Failure of Cantilever due to 
Tension 

are taken, the moment of the weight is equal 
to its value in Ibs., cwts., or tons x by 
its distance or leverage to this point. This 
moment, then, is equal to W x I, where W 
equals the weight and I the length of the 
cantilever. 

Hence, the formula for finding the greatest 
bending moment in the case of a cantilever 



which carries an end load is B M = W I. 
The following is an example. A canti- 
lever with a projection of 10 ft. carries an 
end load of 5 tons. Then B M = W I or 
B M = 5 x 10 - 50 ft.-tons. 

To ascertain the B.M. at any intermediate 




Fig. 75. Failure of Cantilever due to 
Compression 

point it is only necessary to apply the same 
principle namely, multiply the weight by 
its leverage to the point at which the moment 
is to be found. Take the same \\eight and 
projection as in the last example, and assume 
that it is necessary to calculate the B.M. at 
a point 5 ft. from the extreme end. Then 
it will be seen that the bending tendency at 
this point is less than that at the support, 
as, although the weight remains the same, 
the leverage has diminished, and is now only 
5 ft. instead of 10 ft., as Fig. 76. The 
B.M. will now equal 5 tons x 5 ft. = 25 
ft.-tons. 

A rule may now be deduced as follows : 
The bending moment at any point in a 
cantilever which carries a single load at its. 
outer end can be found at any point by 



- 5 



I'iO-O 




Fig. 76. Bending Moment in Cantilever 

multiplying the load by its distance to that 
point. 

It will be seen that one factor in the calu- 
lations always remains the same namely, 
the weight, and it is only the distance or 
leverage that varies. It has been shown 
that the greatest B.M. occurs at the support,, 
and it will be obvious that there is no bend- 



STRESS SIMPLY EXPLAINED 



69 



ing tendency at the extreme outer end, as 
the weight at this point will have no lever- 
age. Therefore, the greatest B.M. will occur 
at the support, and it will diminish from 
this point to the outer end, where it is nil. 
It is also known that the diminution will be 
uniform, as it is dependent only on the dis- 
tance from the support. Therefore, if the 
B.M. at the support is set up to scale at 
this point and a straight line is drawn 
through to the outer end, a diagram will be 
obtained which will represent the B.M. to 
scale at all points in the cantilever due to 
the end load, as Fig. 77. 

Bending Moment in Cantilever carry- 
ing more than one Concentrated Load. 
In the case of a cantilever which carries 
two or more concentrated loads at different 




Fig. 77. Bending Moment of Cantilever Set 
out to Scale 

points, it is only necessary to apply the 
same principle of multiplying the loads by 
their respective leverages at any particular 
point, and the sum of their values will give 
the bending moment. There are one or 
two points, however, to be borne in mind, 
and the chief of these is the fact that a load 
does not cause a bending tendency at any 
point which is not situated between this 
load and the support. 

An illustration will render this quite clear. 
In Fig. 78 the load represented by W will 
only cause a bending tendency in that part 
of the cantilever marked A, and no bending 
tendency whatever in the part marked B, 
which is not between the load and the sup- 
port, and this is obvious, as the load will 
travel inwards to the support over the por- 
tion A to meet the reaction, and there will 
be no tendency to travel outwards over B, 
as no resistance is offered by the outer end. 

Take an example with two concentrated 



loads and calculate the B.M. at different 
points. This will show how the moments 
are to be found. 

The diagram, Fig. 79, shows two loads of 
2 tons and 4 tons respectively. The B.M. 
at the support will be due to each weight 




Fig. 78. How Centrally-loaded Cantilever 
Tends to Bend 

multiplied . by its respective leverage, and 
these must be added, as they are not opposing 
forces, but forces acting together. 

Then B M at support = (W 1 x 6 ft.) + 
(W 2 x 9 ft.) = 2 tons x 6 ft. + 4 tons x 
9 ft. = 12 + 36. 
L B M = 48 ft.-tons. 

As a weight will not cause any bending 
tendency at a point which is not between 
the weight and the support, W 1 will not 
cause any B.M. between W 1 and W 2 , and 
the only tendency will be due to W 2 , as 
Fig. 80. 

B M under W 1 then equals W 2 x 3 ft. = 
4 tons x 3 ft. =12 ft.-tons. 

Bending Moment in Beam carrying 
Distributed Load. Concentrated loads 
only have been dealt with up to the present, 
and although the same principles apply in 
the case of distributed loads a few brief 



- 2 TONS WMTON5 

4 



6-0'- 



Fig. 79. Cantilever with Two Concentrated 
Loads 



notes should be of value, more especially as 
distributed loads are more generally met 
with in practice. A common case that has 
to be calculated is that of a beam carrying 
both concentrated and distributed loads, and 
an example of this nature presents great 
difficulty to one who is not well versed in 
structural design. 



REINFORCED CONCRETE 



As a first example, consider a beam which 
carries a uniformly distributed load over its 
whole length, as shown in Fig. 81. Then the 
total load will be the length multiplied by 
the weight per foot run. This equals 10 ft. 
x 1 ton per foot = 10 tons. The reactions 




The greatest bending moment then equals 
W I [ W I 
"2" : 2 ' 2 I 
Wj Vfl Wj 

~T ~ IT = "8" 





i V 

W 
2 


1 1 

f W 

J 


; 4 




t J 




1 








?i 


hiq 




7 


'//" 

P 


\ z ^~ 1 



Fig. 80. Bending Tendency in Cantilever with 
Two Concentrated Loads 



will obviously be equal, and must together 
equal the total weight, which may be 
expressed as W; Then R 1 and R 2 each = 

W 

-o-. The bending moment at any point is 

equal to the reaction multiplied by its dis- 
tance to that point, minus any weight 
situated between the reaction and the point 
at which the bending moment is to be found 
multiplied by its distance to the same 
point. The greatest bending moment will 
be at the centre of the span, and upon 
reference to Fig. 82 it can be seen what 
values have to be taken. 

W 

There R 1 equals -^ t and this acts upwards 

I 
with a leverage of ~ at the point F, which is 

the centre of the span. There is, however, 



/ 



LOAD - I ion pec POOJ 



t-\o'-o- 



p' I 

Fig. 81. Beam with Uniform Distributed 
Load 



Fig. 82. Distributed Load Acting through 
Centre of Gravity 



This is the formula for the greatest bending 
moment in the case of a beam which carries 
a uniformly distributed load over its whole 
length. To work out the example in Fig. 81 
it is only necessary to apply the formula as 
follows : 

Cr 13 M = Q = ~ = ~TT~ = 12'<> 

o o o 

ft.-tons. 

Let it be now assumed that the bending 
moment is required to be calculated at a 
point 3 ft. from abutment A. 

Then the length of the lever arm is reduced 
to 3 ft., and in addition the load acting 
downward is reduced to 3 tons with a lever- 
age to the point F of 1 ft. 6 in., as in Fig. 83. 
The bending moment equals : 

3 JONS 




Fig. 83. Bending Moment at Intermediate 
Point 



a load on the lever arm which is acting 
downward, and this load is equal to 

-Q-, and being uniformly distributed, it can 

be considered as acting at it 3 centre of 
gravity, which will be situated at a distance 

equal to -r from point F. 



(5 tons x 3 ft.) - (3 tons x 1-5 ft.) 
= 15 ft.-tons 4'5 ft.-tons 
= 10-5 ft.-tons. 

Although the reaction, which is one of the 
factors in the calculations for any inter- 
mediate point, remains the same, the lever- 
age diminishes towards the support, and in 
addition to this, the weight acting down- 



STRESS SIMPLY EXPLAINED 



ward, together with its leverage, diminishes 
as the lever arm of the reaction becomes 
less. The diminution of the bending moment 
from the centre of the span to the support 
is therefore not regular, as in the case of a 
central concentrated load, but actually 
varies in a compound ratio. If the bending 
moment at the centre be set up to scale 
over the beam, then the parabolic curve, 
as shown in Fig. 84, will give the bending 
moment at all intermediate points. With 
regard to a combination of distributed 
and concentrated loading, an example is 
given in Fig. 85, and, if this is explained, 
no difficulty should be experienced with 
other examples, as the principles are 




Fig. 84. Bending Moment on Beam Set up 
to Scale 



always the same, and the procedure is 
quite simple. 

F,irst calculate the reactions : 

K 1 = 

6 tons (distributed load) x 9 ft. + 8 tons x 4 ft. 
12 ft. 



E 1 = 



54 +32 
12 



--S- 



under the load of 8 tons is to be found by 
working from abutment A, then : 
B M = (7J tons x 8 ft.) - (6 tons x 5 ft.) 
B M = 57J ft.-tons - 30 ft.-tons = 27 ft.- 
tons. This can be checked by working from 
abutment B. Then : 

B M = 6 tons x 4 ft. = 27 ft.-tons. 



I TON PER fOOT 




Fig. 85. Beam with Concentrated and 
Distributed Loading 

The bending moment at any other point can 
be found in a similar manner, and, if neces- 
sary, a diagram set up showing the value at 
all parts of the beam. 

Bending Moment in Cantilever Carry- 
ing Distributed Load. In the case of a 
cantilever carrying a uniformly distributed 
load over its whole length, the greatest 
bending moment occurs at the support, and 
the method of finding its value is very simple.- 

The portion of the weight at the extreme 
outer end of the cantilever will have a lever- 
age equal to I, while the portion of the weight 
at the extreme inner end will have a leverage 
equal to nil. The mean leverage may there- 



Ws 




i 

i 


/ 1 TON PE2 fOOJ 




J > 




i 




m 


~- 4-z'-6'-H 


5-0" 







Fig. 86. Cantilever Carrying Distributed 
Load 



R 2 = 



R 2 = 



8 tons x 8 ft. + 6 tons x 3 ft. fore be assumed to be ^ which represents 

12 ft. distance from the support to the centre of 

64 + 18 gravity of the weight. The formula for the 

~~12 greatest bending moment then becomes : 

W x 2 = ~2~ 
In the exampls in Fig. 86, the greatest 



82 



= jTj = 6 tons. 
Assume that the bending moment at F 



REINFORCED CONCRETE 



bending moment is equal to the weight 

I 
of 5 tons multiplied by v>, which is 2 ft. 6 in. ; 

this then gives a moment of 5 x 2 ft. 6 in. 
= 12J ft.-tons. 




Fig. 87. Finding Bending Moment at Inter- 
mediate Point in Cantilever 

In the case of the bending moment at any 
intermediate point, it is only necessary to 
consider that portion of the weight situated 
between the point and the extreme outer 
end. As an example, assume that the bend- 
ing moment is to be calculated at the point 
F in the cantilever shown in Fig. 87. Then 
the weight to be considered is 3 tons, and 




Fig. 88. Bending Moment in Cantilever Set 
out to Scale 

acting at its centre of gravity it will have a 
leverage of 1 ft. 6 in. to the point F. The 
bending moment = 3 tons x 1 ft. 6 in. = 
4J ft.-tons. The moment varies in a com- 
pound ratio, as in the case of a beam, owing 
to the weight diminishing, because the point 
at which the moment is found is moved out- 



ward from the support, and at the same 
time the lever arm also decreases. 

If the bending moment at the support is 
set up to scale as illustrated in Fig. 88, a 
diagram can be drawn to show the bending 



I TON RLE FOOT 

3 TON5 



TONi 



IZ'-O" 



Fig. 89. Cantilever with Combined Distributed 
and Concentrated Loading 

moment at all points in the cantilever due 
to a distributed load. 

Combined Distributed and Concen- 
trated Loading on Cantilever. With re- 
gard to a combination of distributed and 
concentrated loading on a cantilever, an ex- 
ample is given in Fig. 89, and the greatest 
bending moment will be calculated as an 
illustration of the method to be employed. 
It will be seen that three loads are carried, 
namely, a load of 3 tons 5 ft. from the sup- 
port, a load of 5 tons at the extreme outer 
end, which is 12 ft. from the support, and a 
distributed load of 12 tons, which can be 
considered as acting with a leverage of 6 ft. 
The greatest bending moment will therefore 
be: 

(3 tons x 5 ft.) + (5 tons x 12 ft.) + (12 tons 

x 6 ft.) 

= 15 ft.-tons + 60 ft.-tons + 72 ft.-tons 
= 147 ft.-tons. 

The bending moment at any other point 
can be found by considering only that por- 
tion of the loading which is situated between 
the point and the outer end of the cantilever. 

MOMENT OF INERTIA 

The moment of inertia is extremely import- 
ant in all cases of advanced calculations. The 
expression is used to define the relative 
values of sections of different shapes. From 
the moment of inertia, the section modulus 
and moment of resistance (two terms that 
will be explained later) can be found. 

Explanation of Moment of Inertia. 
The moment of inertia of a section is an 
indication of the strength of that section. 
It is a constant value, and depends on 
the shape of the section. The area alone 



STRESS SIMPLY EXPLAINED 



73 



is no indication of the strength of a section 
from a structural point of view. Two 
sections of the same area do not necessarily 
have the same power to resist stress. For 
example, place a piece of timber, 9 in. by 
2 in. in section, as a beam over a certain 
span say 10 ft. it will have a certain 
amount of power to resist stress, or, in other 
words, a certain amount of power to remain 
" inert," which power can be expressed as its 
*' moment of inertia." The timber will have 
two values, according to the Way in which 
it is placed. In the form of a beam 9 in. 
deep and 2 in. wide, it \vill be much stronger 
as a weight-carrying member than as a beam 
2 in. deep and 9 in. wide. This can be 
proved by a practical demonstration quite 
easily, or by the theory of the moment of 
resistance for a rectangular beam, to be 
given later. 

The reason for this difference in strength 
with the same section is entirely due to the 
disposition of the fibres in relation to the 
neutral axis of the section, which is different 
in each case. (See page 3 for an explana- 
tion of what is meant by the term " neutral 
axis.") In the first instance, the beam was 
9 in. deep, and therefore some of the fibres 
were situated at a distance of 4i in. from the 
neutral axis, whereas in the second instance 
the beam was only 2 in. deep, and therefore 
the extreme fibres were only 1 in. from the 
neutral axis. Now the stress in a beam is 
nil at the neutral axis, and it increases in 
intensity as it gets farther away from the 
neutral axis. This fact allows the fibres that 
are at a distance from the neutral axis to be 
utilised to their fullest capacity, whereas 
those adjoining the neutral axis cannot be 
called upon to exert their greatest resistance. 
Again, it will be shown later that the lever 
arm of the tensional and compressional areas 
is increased as the depth is increased, and 
this greatly increases the value of the resist- 
ance offered. 

These remarks should suffice for the pre- 
sent to show that the area of a section alone 
is no criterion as to the strength of the 
member, and that the same section will 
possess two different capacities according to 
the way in which it is placed. 

It will now be understood why the two 
values for the moment of inertia of a section 
are known as the " least moment of inertia " 
and the " greatest moment of inertia," 
according to the neutral axis around which 
the " moment " has been taken. In the 



case of a section which is square or circular 
there will only be one value for the moment 
of inertia, as the axis will be at the same 
distance from the extreme fibres in both 
cases, except in the case of the square 
section being placed on one edge, which is 
so unusual in structural work that it need 
not be considered for the present. The 
moment of inertia in value \vill be dependent, 
in every case, on the area of the section, and 
the disposition of that area in relation to the 
neutral axis of the section. 

Calculating Moment of Inertia. The 
method of finding the moment of inertia 
of a plane figure is based on the following 
principle. The section or plane surface is 




Fig. 90. Calculating Moment of Inertia of 
Simple Rectangular Section 



imagined to be divided into an infinite num- 
ber of thin layers, and the area of each layer 
is multiplied by the square of the distance 
between the centre of gravity of the layer 
and the axis ; the sum of all these products 
is the moment of inertia of the plane surface 
with respect to that axis. 

An attempt should now be made to calcu- 
late the moment of inertia of a simple rect- 
angular section as given in Fig. 90. Assume 
that the section is 12 in. deep and 6 in. wide, 
and the moment of inertia is to be taken 
about the axis x x. Also assume that 
there are three layers on each side of the 
axis 2 in. thick. Then the moment of 
inertia will equal 
[(6 x 2 x 5 2 ) + (6 x 2 x 3 2 ) + (6 x 2 x I 2 )] 

x 2 = [300 + 108 + 12] x 2 = 420 

x 2 = 840. 
The presence of fifty layers on each side of 



74 



REINFORCED CONCRETE 



the axis, instead of three, could be assumed, 
and thus a great amount of time could be 
tediously spent in calculating the " moment." 
A formula has, however, been devised for the 
moment of inertia of all the simple sections, 
and the formula will be given for each case. 



about the axis x x than about Y y (Fig. 
99). The formula will be I = & ^ 



6 x 12 3 - 2 x 2-75 x 10-5 3 



12 
= 333421. 



To calculate the moment about the other 



p b- 


* ( -D f 


r 
d 

1 


I 

L 


/ \ 


3 W 


Fig. 9 
I - b 


i 

d 8 . ! _ 

12 

b >| 


Fig. 92 Fi ' 93 
6~d 3 - 6' d' 3 I = '7854 r 


12 






1 


t *~r n^ 

'igs. 91-97 ^ 

mon Sections 

^ ., . int.-u.TRAi AXIO -k 


T7 
t 


I F 
Com 

Ll \ 


Foments of cf 


Fig. 96 
j _ 6 d 8 - 26' d' 3 


Inertia ^ J[c' 


Fig. 97 




12 




i. = 



Fig. 94 

7854 (r 4 - r' 4 ) 




I = j | b d 3 + b' d' 3 
- (6'- b)d" 3 \ 



I -I {&*- (b -i) (d -c) 3 
+ b' d' 3 - (b' - t) (d' - c') 3 [ 



axis, place the joist so that it is 6 in. deep and 
12 in. wide, as Fig. 99. The inertia will be 
flanges web 

75 x 6 3 x 2 + io-5 x -5 3 



-b' J 
Fig. 95 



o a 3 

Now the formula for a rectangle is - ^-, 

12 

fe 4 
and for a squara j^ an( i so t^ 6 true value 

of the inertia moment for the section given 



in Fig. 90 will be I = ~~- 

6 x 12 3 
-12 - 864 ' 

It will be seen that there is some difference 
in the result obtained by the formula and 
that obtained previously, this being due to 
the fact that the number of layers in the 
first instance was taken at three only. The 
moments of inertia of the common sections 
are as given by Figs. 91 to 97. 

In (ach case these formulae give the 
least moment of inertia in the positions 
shown, and as an example a rolled steel 
joist will be taken and the moment of 
inertia calculated (see Fig. 98). 

The moment of inertia will be more 





1 n " ^('JLUy. 

L K - A* 










_ 1 






^ 




7 


r ) 












d 


-"72" 


x- 




i 






v 








A 






t ^> 


/ 






i 








f 


' 


i 


d 


- lO'J 









Fig. 98. Calculating Moment of Inertia of 
Rolled Steel Joist 



STRESS SIMPLY EXPLAINED 



75 



SECTION MODULUS AND MOMENT OF 
RESISTANCE 

The terms " section modulus " and 
" moment of resistance " are often used 
indiscriminately, but although they are 



are equal, opposite, and parallel forces form- 
ing a " couple." 

Take one-half of the beam and imagine 
the forces to be acting upon it, as shown in 
Fig. 101. Then the rotating tendency 




Fig. 99. Calculating Least Moment of Inertia 
of Rolled Steel Joist 

closely allied, they are distinct terms, and 
have different values. 

The section modulus can be found by 
dividing the moment of inertia by half the 
depth of the section. 

The moment of resistance can be found by 
multiplying the section modulus by the 
modulus of rupture of the particular material 
under consideration. 

Thus, the section modulus is a fixed 
quantity for any given section, whilst the 
moment of resistance depends on the nature 
of the material employed. 

The bending moment or tendency to cause 
failure in a member must be equalled or 
exceeded by the moment of resistance of 
that member ; and to illustrate this fact, 
and also how the section modulus is obtained 
and from this the moment of resistance 
will be the next step. 

Calculating Section Modulus. It has 



L 



Fig. 100. Rotating Tendency of Beam, caused 
by Reaction and Weight 

already been shown that the reaction and 
the weight on a beam causes a certain 
rotating tendency, as indicated in Fig. 100. 
It has also been shown that the value of 
these external forces on the beam is equal 
to the amount of one force multiplied by 
its distance from the other force, as they 



Fig. 101. Forces Acting upon One-half 
of Beam 

caused by the external forces must be 
equalled by the internal forces having a 
rotating tendency in the opposite direction. 
This rotating tendency of the internal forces 
is offered by the compressional and tensional 
resistances of the beam, which are equal to 
one another, opposite in direction, and 
parallel, as shown in the diagram by c and T. 
Then these two resistances form a " couple," 
and their value to resist the bending moment 
will equal the amount of one resistance 
multiplied by their distance apart. The 
method of ascertaining this value will be 
shown. 

It will be seen that the rotating tendency 
of the reaction and weight is in a " clock- 
wise " direction, whilst the tendency of the 
internal forces is " anti-clockwise." The 
direction of the tension and compression 
arrows, shown on the diagram, can be proved 
in the following manner. The firm arrows 



N - 


w 
a 4 b 




1 


-f? 


-N 




1 


' v \ 


C I d 


W 1 

'/ 



Fig. 102. Calculating Section Modulus : 
Beam Centrally Loaded 

indicate the fibres on the half of the beam 
under consideration, and the dotted arrows 
show those on the other half of the beam. 
The top fibres are being compressed or pushed 
together, and therefore the tendency of the 
resistance is to pull the fibres apart as shown, 
and vice versa in the lower part of the beam. 



7 6 



REINFORCED CONCRETE 



The amount of the rotating tendency of 
the internal forces can be reasoned as 
follows. Assume that a beam carries a 
central load, as Fig. 102, and mark a piece 
of the beam abed as shown. The tendency 




Fig. 103. How Centrally Loaded Beam tends 
to bend 

of the load is to cause the beam to bend in 
the middle, and this is shown in an exagger- 
ated manner in Fig. 103. Now mark on the 
beam the same portion abed as shown on 
the previous diagram. In order to do this, 
begin by marking the length n n on the 
neutral axis, as it is known that no change 
has taken place in the length of the fibres 
at this point, there being no compression or 
tension. Through these points n n draw the 
lines a c and 6 d, which must, of course, 
radiate to the same centre as that from 
which the curved beam was struck. It will 



D 

-N 
12." 



VARYING INTENSITY 

OF t)JKLt>$ 

Fig. 104 




parallel to the line a c, as shown on the 
diagram in Fig. 103, the distance 6 a; "will 
equal the amount of compression that has 
taken place in the extreme fibres at the top 
and the distance x d will indicate the amount 
of tension that has taken place in the extreme 
bottom fibres. The stress in the extreme 
fibres must not exceed the resistance of the 
material, as otherwise failure would result. 
This will demonstrate the fact that as the 
extreme fibres only can be stressed to the 
safe resistance, the full value of the other 
fibres can never be realised, and the value 
of each layer of fibres decreases as it 
approaches the neutral axis. 

This principle can now be applied to the 
section of a beam and the value of the com- 
pression or tensional areas ascertained. Take 
the section of a beam 12 in. by 6 in., as shown 
in Fig. 104, where the intensity of the stress 
is indicated by the graduation of the lines, 
which become fainter as the stress becomes 
less. Assume also that the limiting stress 
on the extreme fibres is 5 tons per square 
inch. Then the stress at the neutral axis 
is nil, and the mean stress on the fibres is 
2^ tons per square inch. The resistance 
must be equal to the stress if there is to be 
equilibrium, and therefore the resistance of 
the compressional area will be 6 in. x 6 in. 
x 2 tons per square inch = 36 x 2| = 
90 tons. 



Of 6RAVIJY 




Fig. 105 



Fig. 106 



Figs. 104 to 106. Calculating Section Modulus of Beam 



then be seen that a c and b d are no longer 
parallel to one another as in the original 
beam, or, in other Words, the distance 
between a and b has become shorter owing 
to compression and the distance between 
c and d has become longer owing to tension. 
Now, if a line a; a; be drawn through n 2 



Next, instead of assuming that the stress 
on the fibres varies in intensity from the 
outer edge to the neutral axis, assume that 
the intensity is the same, but the area over 
which it acts varies from the outer edge to 
the neutral axis as shown by the shaded 
triangles in Fig. 105, and it is found that the 



STRESS SIMPLY EXPLAINED 



77 



same result is given, with the same sectional 
area of beam. Here the shaded triangle 
which shows the compressional area is equal 
to one- quarter the sectional area of the 

beam, or, in other words, -j-. Afterwards 

multiply this by the uniform maximum in- 
tensity of 5 tons per square inch, then 

6 x 12 

j x 5 tons per square inch 



72 



360 



= 90 tons 



bd 
as before. This area -^ is called the 

equivalent inertia area, and it simplifies 
the calculations to consider the beam as 
offering its resistance in this way. 

There are two resistance areas, namely, the 
compressional and the tensional, and it will 
be seen that they are equal. They repre- 
sent the internal forces which are opposite 
in direction, and their value to resist stress 
is the amount of one multiplied by its dis- 
tance from the other, as they form a 
" couple." The distance apart of the two 
forces will be the distance between their 
centres of gravity as they are represented 
by triangles, and the centre of gravity of a 
triangle is situated at a distance of two- 
thirds of its height from the apex, as shown 
in Fig. 106. The centres of gravity are then 
d 



each f ~ from the neutral axis, and their T nen M R = 



The section modulus is usually expressed 
in formulae by the letter Z. 
Calculating Moment of Resistance. 

Having ascertained the " section modulus " 
and followed the principle of the action of 
the internal forces, the actual resistance of 
these forces, which not only depends on the 
area and leverage, but also upon the strength 
of the material, has to be considered. 

Up to the present, no account has been 
taken of the strength of the material, and 
therefore the section modulus for any shape 
section will be constant, no matter what 
material is employed, and this is a great 
consideration. 
b d 2 

Thus -JT~ i s tne section modulus for a 

rectangular section, and the moment of 
resistance can be found from this, according 
to the strength of the material employed, 
by multiplying the section modulus by the 
ultimate or safe strength of the material 
per square inch, according to whether the 
actual or safe resistance is wanted. 

As an example, ascertain the safe resist- 
ance of a rectangular steel beam 8 in. x 2 in. 

b d 2 
Then M R = - x 7 tons, or M R = 

j 7 79 

^ , where / equals the limiting stress on 

the fibres, which is 7| tons per square inch. 
2 in. x 8 in. x 8 in. x 7| tons 



distance from one another is ~ +f~9 

which equals -f d. 

The distance f d then equals the lever- 
arm of the internal forces. The area of one 

force = . and the area multiplied by the 

lever-arm will give the value of the section. 
This can be expressed as 

bd 2bd 2 bd 2 

~~ ~~ 



The value 



b d 2 



then gives the value of a 



rectangular section, or, in other words, its 
" section , modulus." It was stated pre- 
viously that the moment of inertia divided 
by half the depth of the beam = the section 
modulus. The moment of inertia for a 



rectangular beam is y^-. 



bd 2 
= -7T- as before. 



12 



bd* d 
Then -J2- -*- 2 



M R = 160 in.-tons. This, of course, must 
equate with the bending moment to produce 
equilibrium. 

It has been seen that the moment of inertia 
divided by half the depth of the section gives 
the section modulus, and the section modulus 
multiplied by the limiting stress on the fibres 
gives the moment of resistance. This is 
expressed in the following formula, which is 

one greatly used in practice : MR = / -, 

where / = limiting stress on the fibres, I = 
moment of inertia, and y = half depth of 
section. Upon working out the example 
just given by this method, the same result 
will be obtained. 

CALCULATING SAFE LOAD ON BEAM 

The following example should assist the 
student to understand the application of 
formulae already given. Find the central 
load that can be safely carried over a 
span of 16 ft, by a rolled-steel joist 12 in. 



7 8 



REINFORCED CONCRETE 



x 6 in., with flanges and web 1 in. thick. 



MB*/= ,/ = 7|tons, 1 = 

y 1Z 1J 

6 x 12 x 12 x 12 2 x 2| x 10 x 10 x 10 

~~12~ 12~~ 

- 864 - 416-6 = 447-4 

d 12 

v = -R = -TT = 6 in. 



then M R = 



7-5 x 447-4 
~6~ 



M R = 559-25 in.-tons. 



w I 



But B M - M R, and B M = -^ for central 

load. 

w I 
Therefore -r- = 559-25 in.- tons 

I = 16 x 12 = 192 in. 
^^= 559-25 



559-25 x 4 






WEI6HT 



w = 11-65 tons. 

This method is absolutely accurate, and 
is not approximate, as would be the case 
with an empirical for- 
mula. The student is 
strongly advised to 
work out several ex- 
amples himself, which 
will familiarise him 
with the use of the 
formula, and impress 
it upon his memory. 
A good method is to 
select several rolled- 
steel joists from a 
manufacturer's list, 
calculate the safe loads 
for various spans, and 
then check the results 
by reference to the 
table of s.afe loads 
for the various joists 
invariably given in 
the list of sections. 




REACTION 



Fig. 107. Strut 
under Load 



COLUMNS AND 
STRUTS 



The theory of long 
columns and struts 

unfortunately is in an unsatisfactory con- 
dition, as there is a great difference of 
opinion as to the best formula to use ; and 
as the formulae in general practice give 
very varying results, the student is often 
confused and doubtful as to the best course 



to adopt; Most of the formulae in use are 
of a more or less complicated nature, and 
practically all are empirical formulae, con- 
taining some factor which has been derived 
from experiment, and which must be taken 




Fig. 108. Diagram Indicating Stress in Part 
of Strut 

for granted. Unlike a tension bar, a com- 
pression bar which is under a certain stress 
is apt to bend, and this bending tendency 
has to be resisted, in addition to the com- 
pression proper, as shown in Fig. 107. 

Theoretically, with the stress passing abso- 
lutely through the centre line of the strut 
there is no tendency to bend, but practically 
this condition is never fulfilled, and the 
slightest deviation from the exact centre 
will cause a bending stress. The diagram 
in Fig. 108 shows a piece of the strut at 
A B enlarged, and the arrows indicate the 
stress. The horizontal line represents the 
compression proper, with an amount equal 
to x, while the sloping line indicates the stress 
due to the bending tendency, and this, of 
course, will vary in intensity at different 
points in the section ; the edge of the strut 
at B will be under great compression, while 
the fibres at A will be in tension. Now the 
maximum intensity of compression will be 
at B, where the stress will equal x x y, 
while at A the stress will be at a minimum, 
and will be equal to a; y, and if y is greater 
than x, then the fibres at this point will be 
in tension. It is clear that the greatest stress 
x + y must not be greater than the safe 
resistance to compression of the material. 
The principal difficulty occurs in the deter- 
mination of the bending tendency. 

Struts and columns are usually divided 
into " long " and " short " compression bars, 
and in the short bars the calculations are 
based on the assumption that the failure is 



STRESS SIMPLY EXPLAINED 



79 



by direct crushing only, while long bars are 
those which fail by bending. A sudden 
change has been assumed, although, of 
course, this does not actually happen. The 
following is the ratio between the least 
diameter and the length when the bars cease 
to be considered as short. Wood, 20 ; 
wrought-iron and steel, 10 ; cast-iron, 5. 
These ratios will be found to vary greatly 
according to different authorities. 

The formula chiefly used in this country 
for long struts is that known as " Gordon's 
Formula," which is as follows : 

R_ = Ar ' -. where 



a = a constant found by experiment. 

d the least diameter. 

A = area of the cross section. 

r e = safe resistance to compression of the 

material. 

I = length in same units as d. 

R e = total pressure in same units as r c . 



Material 


CVoss 
Section 


Values of a 


r c 


* 
" 2 1 
1^ 


ITS 
14 


! 

23- 


TIMBER . . 


Rectangular 1 
Circular . . j 


4 
250 


i 

250 


i 

100 


13 

cwt. 


WROUGHT- 
IRON 


Rectangular 
Circular . . 
Solid 
Hollow . . 


4 


1 


1 




2500 


2500 


1000 




LT+ I 

n J 


4 

900 


1 

900 


1 

360 


-4 tons 




I U j 










STEEL 


do. 


do. 


do. 


do. 


1\ tons 


CAST-IRON 


Circular 


1 


1 


1 






Solid 


100 


400 


160 






Circular 


1 


1 


1 






Hollow . . 
Rectangular 


200 
3 


800 
3 


320 

3 

640 


: 8 tons 


400 


1600 




X shaped . . 


3 

200 


3 

800 


3 

320 


/ 



In r e a factor of safety of 4 only is allowed, 
and therefore, in the case of timber and cast- 
iron, it will be advisable to use a higher 
factor in important work, and this will 
reduce the value of r^ 



SHEARING STRESS 

Before studying calculations for the value 
of the shearing stress under different con- 
ditions of loading, it will be advisable to 
explain what the expression " shearing 



DOTTED 



BEAM 











1 



























v\ 
















'(/' 


i 




1 











Fig. 109. Beam, under Vertical Shear, 
Assumed to Consist of Separate Blocks 



stress " means. This stress is actually a 
sliding tendency caused by the opposition 
of the load and the reaction, and as the 
beam or cantilever is the medium through 
which these two can oppose one another, 
the beam is subjected to this tendency both 
vertically and horizontally. As an illus- 
tration of this, let it be assumed that it is 
possible to divide a beam up into a cer- 
tain number of parts by cutting vertically 
through it, so that each block is separate 
from the adjoining block, and yet it is pos- 
sible to put the beam in position and apply 
the load. Then the load, in travelling along 
the beam toward the abutments would tend 
to push down each block and cause it to 
slide past the adjoining block as illustrated 
in Fig. 109. This does not actually take 
place in practice, as the beam would be 
homogeneous ; but the tendency is pre- 
sent, and must be provided for. This is 
known as vertical shear. There is also 



DOJTtD LlNEt) 6HtW 




Fig. 110. Beam, under Horizontal Shear, 
Assumed to consist of Separate Planks 



a tendency to shear in a horizontal manner, 
and this is illustrated in Fig. 110, where the 
beam is divided into a number of horizontal 
parts or planks ; upon the load being 
applied, there is a tendency for the beam 
to assume the form shown in an exaggerated 



REINFORCED CONCRETE 



manner. As the ends of the planks coin- 
cided in the first instance, and after the 
load has been applied they are no longer 
coincident, it is obvious that they have 
slid past one another, thus forming a hori- 

W- 10 TOKS 



Fig. 111. Shearing Stress in Cantilever which 
carries Concentrated Load at Outer End 



zontal shearing action. At all points in 
the beam the vertical and horizontal shear- 
ing stress are equal, and they will be found 
to be at the maximum at the support. The 
distribution of the shearing stress over the 
area of the cross section is explained in the 
next chapter and need not be considered 
here. The amount of shearing stress is not 
due to any question of leverage, but is due 
to the amount and nature of the loading, 
and not to the span, as in the case of the 
bending moment. The conditions under 
which it is likely to become a serious con- 
sideration are those where a heavy load 
has to be carried over a short span, as in 
this case the bending moment will be com- 
paratively small the leverage being small. 



Dl6TRi&ujED LOAD- (TON pte 
FT ^UH 




112. Shearing Stress in Cantilever which 
carries Uniformly Distributed Load 



The possibility of failure by shear is fre- 
quently ignored on account of the fact 
that the amount of material required to 
resist the bending moment is usually such 
that ample provision for shear is given. 
In the case of heavy loads over a short span 



this does not always follow, and consequently 
the shearing stress must be calculated for 
as well as the bending moment. With 
regard to the distribution of the shearing 
stress over the length of the cantilever or 
beam, it must be fully realised at the out- 
set that this stress is due to the opposition 
of the load and reaction, and consequently 
the value of the shear at any point can only 
be equal to the load that is travelling 
through that point to reach the abutment. 
As a first example, consider a cantilever 
that is carrying a concentrated load at its 
outer end. Then at the support the shear- 
ing stress will be equal to the reaction, as at 
this point the total load will be opposed by 
the total reaction, and the opposition is 
therefore at the maximum. If any inter- 
mediate point in the cantilever is considered, 
it will be seen that the shear is still equal to 



5TON6 5 TONS 

I DISTRIBUTED LOAD I 
if I TON pEEf FT RUN. I 




^__?TONi 

JTONi 

I 

5TOITS 

- J r_ -i 



i 



Fig. 113. Shearing Stress in Cantilever which 
carries Uniformly Distributed Load and 
Two Concentrated Loads 

the total load, as in passing outward from 
the support no load is passed over or left 
behind, as it were, until the extreme outer 
end is reached. The shearing stress at all 
points will therefore be as indicated in 
Fig. Ill, where the length of the vertical line 
at any point equals the shear at that point. 
If W T= 10 tons, then the shearing stress at 
the support or any intermediate point equals 
10 tons. 

In the case of a cantilever carrying a 
uniformly distributed load, the shearing 
stress at all points is somewhat different, as 
will be seen on reference to Fig. 112. In 
this case the shear will uniformly diminish 
from the support to the outer end, where it 
will be nil. In passing out from the sup- 
port, it will be obvious that the load will 
be gradually passed over, as, for example, 



STRESS SIMPLY EXPLAINED 



81 



at a point 1 ft. from the support 1 ton will 
be left behind and the opposition of 10 tons 
reduced by this amount, giving a shearing 
stress equal to 10 - 1 = 9 tons. When 
5 ft. has been passed over, then 5 tons will 
be left between this point and the support, 
and the shear will equal 10 tons - 5 tons = 
5 tons, as shown in the diagram. 

It has been stated that the shear at any 
point can only equal the load that is passing 
through that point on its way to the sup- 
port, or, in other words, in a cantilever the 
shear at any 'point is equal to the load between 
that point and the other end. Apply this rule 
to a cantilever carrying both distributed 
and concentrated loads ; it will be shown 
how the shear is obtained. The example in 
Fig. 113 shows a cantilever carrying a 
uniformly distributed load of 1 ton per foot 
run over a length of 10 ft., together with 




5 



Fig. 114. Shearing Stress in Beam which 
carries a Central Concentrated Load 



two concentrated loads of 3 and 5 tons res- 
pectively. Beginning at the outer end and 
passing inward, and applying the rule that 
the shear at any point equals the load be- 
tween that point and the outer end, we shall 
have placed a distributed load of 2 tons 
between us and the outer end by the time we 
have passed over 2 ft., and consequently 
there will be a gradually increasing shear 
from nil to 2 tons ; and upon passing over 
the 2 ft. mark the centre of gravity of the 
concentrated load of 3 tons will be passed 
and placed between us and the outer end, 
and the shear is therefore suddenly increased 
by 3 tons, giving a total of 5 tons. The 
next 5 feet give a gradually increasing shear 
of 5 tons, which must be set downwards on 
the diagram, and immediately on passing 
the line of the second concentrated load 
another 5 tons is added, giving a shear of 
15 tons. From this point to the support 
we pass over a distance of 3 ft., and gradu- 
6 



ally leave behind the remaining portion of 
the distributed load, equal to 3 tons, thus 
giving a total shear at the support of 18 
tons, which is equal to the reaction at the 
support. This is clearly illustrated on the 
diagram referred to. The same principles 
and rule apply to any kind of loading, 
whether partly distributed or partly con- 
centrated, bearing in mind that a distri- 
buted load causes a varying shear, repre- 
sented by a sloping line, and a concentrated 
load causes a sudden increase in the shear 
equal to the amount of the load. 

Beams require a little more consideration 
on account of there being two supports, 
and consequently a portion of the loading 
travels one way and is opposed by one re- 
action and the remainder travels in the 
opposite direction and is opposed by the 
other reaction. We have, therefore, a shear 
towards the left-hand abutment, caused by 
the opposition of R 1 , and a shear toward the 
right-hand abutment caused by R 2 , and the 
shear at either support will equal the re- 
action at that support. In the case of a 
beam carrying a central concentrated load 
only, as illustrated in Fig. 114, the reac- 

W 

tions will each be equal to -^, which in this 

case is -~- = 5 tons. To distinguish between 

a 

the shear towards the left-hand and right- 
hand abutments, a horizontal line is drawn, 
known as the separation line, and the value 
of R 1 is set up to scale over this line and 
the value of R 2 below as shown. The full 
opposition will exist at all points between 
the support and the centre of gravity of 
the load, as the whole of the load which 
travels to the support will be passing through 
every point in the beam, and there is con- 
sequently no diminution in the shear at 
any point. 

The rule for the shear at any point in a 
beam can be expressed as follows : the 
shearing stress at any point in a beam is 
equal to the reaction at the support minus any 
load situated between that point and the sup- 
port selected. If the load between the point 
and support selected is greater than the 
reaction, then it indicates a shear toward 
the other support. As an illustration of this, 
supposing the shear at a point 7 ft. 6 in. from 
the left-hand abutment shown in Fig. 114 
is to be calculated from R 1 . Then (R 1 - 
the load of 10 tons) = (5 tons - 10 tons) 
= - 5 tons, thus indicating a shear toward 



REINFORCED CONCRETE 



the right-hand abutment which agrees with 
the diagram. In the case of a uniformly 
distributed load on a beam, the shear at 
either abutment will be equal to the reac- 
tion at that abutment, and it will gradually 
diminish to the centre, where it will be nil, 
as illustrated in Fig. 115. An example of 
combined concentrated and distributed load- 
ing is given in Fig. 116, and if this is ex- 
plained no difficulty should be encountered 
by the student in working out other examples, 
however complicated the loading may be. 

As the reaction at the support is equal to 
the shear, it will be necessary to calculate 
this in the first instance. Therefore K 1 will 

equal half the distributed load = 

B 

(3 x 8) + (4 x 6) + (5 x 3) 
5 tons + - 






Loftb - | pn peg 17- KM 




Fig. 115. Shearing Stress in Beam which 
carries Uniformly Distributed Load 

24 + 24 + 15 
5 tons + - - = 5 tons + 6^ 



tons = 11 T S 7, 



10 
tons. 



Working out R 2 = 5 



tons + 



tons + 



(3 x 2) + (4 x 4) + (5 x 7) 
10 

= 5 tons + 



= 5 



6 + 16+35 
10 



10 T 7 ^ tons. E 1 +R 2 = total loading, therefore 
llfV + lOy 7 ^ = 22 tons, which is correct. 
If we now draw a horizontal separation 
line and set up R 1 to scale at the left-hand 
abutment, we shall have the shear at this 
point, and we can proceed to find the shear 
at all points. After passing over the first 

2 feet we have reduced the shear by 2 tons, 
giving 9-^ tons at this point, and directly 
we pass this mark we reduce the stress by 

3 tons, due to the concentrated load leaving 
a shear of 6^ tons. We now gradually 



leave behind a further 2 tons by the time 
the next concentrated load is reached, 
making a reduction to 4j%- tons, and upon 
passing the concentrated load we only have 
T 3 o ton left as shear toward the left-hand 
abutment. As the distributed load equals 
1 ton per foot run, it will be obvious that 
when we have passed over a further distance 
of y'k ft., we shall have no shear to this 
abutment, and it is therefore at this point 
that the separation line will be crossed 
and the shearing stress will be below the 
line, and thus towards the right-hand abut- 
ment. This point of crossing the line will 



III ION 



3 JONS 



2 TO 



LT 



4_ 



5UJED Lc 
PEE- FT " 



i* 



Fig. 116. Shearing Stress in Beam which 
carries Uniformly Distributed Load 
and Three Concentrated Loads 

be 4^ ft. from the left-hand abutment, 
and it is important to be able to calculate 
this in the case of reinforced concrete beams 
when special shear members are provided 
as shown in the notes on this work. In 
continuing along the beam it will be seen 
that after passing the point of crossing the 
separation line a distance of 2 T 7 TT ft. will 
be passed over before reaching the load of 
5 tons, thus picking up 2 T 7 Ty tons, and upon 
adding the 5 tons the shear will equal 7 T V 
tons. The remaining 3 ft. will add a gradu- 
ally increasing shear of 3 tons, making a 
total of lOy 7 ^ tons upon reaching the sup- 
port, and this amount is equal to the re- 
action, and therefore is correct. Fig. 116 
shows the shear at all points in a graphic 
manner, and should assist the student in 
following the explanation. 



The Theory of Reinforced Concrete 



Introductory. The theory of reinforced 
concrete is, unfortunately, in a very unsatis- 
factory state at the present time, and, in 
fact, it may be said that no kind of universal 
agreement has been arrived at. Students 
have no definite course of study that can 
be followed with the surety that diligent 
application will enable them to design and 
calculate the material in a manner consistent 
with a universally recognised standard. The 
desirability of standardising the methods of 
design and calculation is fully recognised by 
the various Institutes who are using every 
endeavour to bring about this reform ; and, 
in the meanwhile, it is incumbent upon all 
writers on the subject to help matters forward 
by adopting the recommendations of the lead- 
ing Institutes as regards notation and general 
methods. 

For the purpose of this treatise, it has been 
decided to adopt the notation recommended 
by the Concrete Institute, this being regarded 
by the writer as the most satisfactory that 
has been compiled. The working stresses 
employed are those contained in the second 
xeport of the Royal Institute of British 
Architects. 

A knowledge of stresses and strains is 
necessary to every student before an attempt 
is made to take up the calculations for re- 
inforced concrete. Therefore, although these 
notes apply to simple theory only, it is 
necessary to assume that the reader has 
mastered the preceding chapter and possesses 
the knowledge referred to ; and only those 
explanations will be given which are essen- 
tial to a proper understanding of the methods 
of deduction. There is a great tendency to 
Tender formulae and explanations very com- 
plicated, with the result that many students 
possessing only a limited knowledge of 
mathematics are unable to follow them. 
Bearing this in mind, the writer has endea- 
voured to delete all unnecessary complications 
a.nd to put the matter in a simple manner, 
which is capable of being understood by all 
readers. The many different symbols used 
in the formulae are apt to look formidable 
at first sight, but the reader is advised to 



make every effort to become familiar with 
them and to endeavour to realise the full 
meaning of each one. 

It is not sufficient to learn several formulae 
and to be able to put them down from 
memory, as this will not enable them to be 
used intelligently ; each one must be under- 
stood and its construction realised. Indeed, 
the user should be able to construct his own 
formulae when he has fully grasped the 
principles of force and resistance. 

The calculations can be divided into two 
sections, namely, (1) the determination of 
the value of the external force or forces, 
and (2) the determination of the necessary 
resistance values to overcome these forces. 

The value of the force to cause stress will 
depend upon the nature, amount, and dis- 
position to the resisting member, and the 
value of the resistance will depend upon the 
shape and size of the member and the 
material of which it is composed. 

DATA FOR CALCULATIONS 

Ascertaining Loads to be Carried. 

There must be some basis upon which to 
found the calculations, and the following 
methods of ascertaining the loads to be 
carried must be taken into consideration : 

(1) The weight of the structure, including 
flooring, plaster, and any applied decoration 
or similar loading. 

(2) The superimposed or accidental load, 
which will vary in amount according to the 
class of building being designed. 

(3) Vibration and shock wherever this is 
likely to occur. This will vary with the class 
of building, as obviously a greater allowance 
must be made in the case of a building 
containing machinery than in the case of 
an ordinary dwelling house. 

In the case of item No. 1 the weight of the 
concrete and steel may be taken at 150 Ib. 
per foot cube. To this must be added the 
flooring material or any other weight, accord- 
ing to circumstances. 

The load per square foot to be allowed, 
as in item 2, may be taken on the following 
basis : 



8 4 



REINFORCED CONCRETE 



lb. 'per sq. ft. 

Ordinary dwelling house, flats, 
hotel bedrooms, and hospital 
wards ..... 75 

Offices, schoolrooms, etc. . . 100 

Theatres, libraries, concert-rooms, 

banks, and shops . . . 120 

Ballrooms and drill halls . . 150 

Warehouses and stores vary ac- 
cording to use . . . 230-670 

Factories and machine shops. These 
will vary greatly, according to 
the number and weight of 
machines, and each case must 
have special consideration. 

Koofs generally .... 50 

The allowance for shock mentioned in 
item 3 will be necessary in the case of build- 
ings which are subjected to greatly varying 
loads, as, for instance, public buildings, 
factories, or workshops, and such allowance 
in these cases should be taken at one half the 
superimposed load as given above for the 
various buildings. In structures where the 
vibration is caused by machinery or heavy 
traffic, as in the case of vault roofs, the 
allowance should be equal to the actual 
superimposed load. 

In the case of columns and piers which 
carry several stories above, a certain reduc- 
tion of the load picked up at the different 
levels is permissible, as follows : For the 
part of the roof or top floor supported, the 
actual superimposed load calculated is taken. 
For the next floor below a reduction of 10 
per cent, is made, and for the floor below 
this a reduction of 20 per cent, is made, and 
so on until the floor is reached at which 
the reduction is equal to 50 per cent. 
Below this level all the floors are taken 
at 50 per cent, below the superimposed 
loads in determining the reactions on the 
columns. 

This reduction must, of course, be used 
with discretion, as it is based on the assump- 
tion that all the floors would not be loaded 
to their full capacity at the same time, and 
therefore, in the case of warehouses and 
some buildings where all the floors may be 
heavily loaded together, it is wide to make 
no reduction whatever. 

Working Stresses. The following work- 
ing stresses are based on the assumption 
that the concrete is of such a quality that 
its crushing strength is at least 1,800 lb. per 
sq. in. after twenty-eight days, and that the 



steel has a tensile strength of not less than 
60,000 lb. per sq. in. : 

lb. per sq. in. 
Concrete in compression in beams 

subjected to bending . . GOO 

Concrete in columns under simple 

compression .... GOO 

Concrete in shear in beams . 60 

Adhesion or grip of concrete to 

metal 100 

Steel in tension .... 16,000 
Steel in compression fifteen times 

the stress in the surrounding 

concrete .... 
Steel in shear .... 12,000 

The resistance of concrete to tension is 
neglected, as its value is small, and, in 
addition, it cannot be considered as a reli- 
able material to take such a stress ; there- 
fore, the steel is calculated to take all the 
tensile stress. The figures given above for 
concrete are based on tests made with con- 
crete made in the proportions of 1 cement, 
2 sand, and 4 hard stone, and should different 
proportions be adopted the stress may be 
taken at one-third the crushing strength 
twenty-eight days after moulding. This may 
be considered a low factor of safety by some 
designers, but it must be borne in mind that 
the maximum strength of the material is 
not developed in twenty-eight days. 

If the steel has a greater ultimate strength 
in tension than 60,000 lb. per sq. in., the 
safe stress may be taken at one half of the 
stress required to reach the elastic limit of 
the material ; but in no case is it advi?able 
to calculate for a greater stress than 20,000 
lb. per sq. in. 

The stress is considered as acting on the 
concrete in a uniformly varying manner, 
while in the case of the steel it is considered 
as uniform over the cross section. 

Elasticity. The question of the relative 
elasticity of the two materials is highly 
important, and although there is some 
difference of opinion on this point, it is now 
generally accepted that the coefficient of 
elasticity for concrete in compression gauged 
as above mentioned is constant and can be 
taken at one-fifteenth that of steel. The 
modulus for concrete is expressed by E,., 
and equals 2,000,000 lb. per sq. in. The 
modulus for steel is expressed by E s , and 
equals 30,000,000 lb. per sq. in. ; therefore 

E 

T/ = 15. It is highly important that the 



THEORY OF REINFORCED CONCRETE 



student should realise this point, as it means 
that at any given distance from the neutral 
axis the stress per square inch on the steel will 
be fifteen times as great as that on the con- 
crete. In other words, the stress can only 
be proportional to the resistance where 
equilibrium is produced, and as the steel is 
capable of offering fifteen times as much 
resistance to stretching as the concrete is to 
compression, it will require fifteen times 
more stress in the steel to produce the same 
effect a^ that taking place in the concrete. 

Spans. The span to be considered in the 
calculations will not, of course, be the clear 
span, but the effective span, and this, in the 
case of beams, may be taken as the distance 
between the centres of bearings. For slabs 
supported at the ends only, the distance will 
be the clear span plus the thickness of the 
slab ; while for continuous slabs it becomes 
the distance from centre to centre of the 
supporting beams. 

Bending Moments. The question of 
bending moments has been fully dealt with 
in the preceding chapter, but it must be 
borne in mind that a slab can only be con- 
sidered as continuous when it is continued 
over three or more spans, and then the spans 
must be equal and the supports at the same 
level. With a uniformly distributed load 
the maximum bending moment may be taken 



as + -r~- at the centre and ^r^ at the 

intermediate supports, where W = total 
weight and I equals span. 

BEAMS WITH SINGLE REINFORCEMENT 

Some little difficulty is experienced by the 
student in determining the moment of re- 
sistance of a reinforced beam, as there are 
two materials to be considered, each having 
a different modulus of elasticity and re- 
sistance to stress. It is necessary, however, 
to be able to determine their moment of 
resistance at any point in order that it may 
be shown that it equates with the bending 
moment at the same point. 

The bending moment represents the value 
of the external forces to destroy the beam 
by tension or compression, the former taking 
place on the lower part of the beam and 
the latter on the upper, where the beam is 
supported at the ends only. If the beam is 
fixed at the ends, then the beam will be 
called upon to resist compression in the lower 
part and tension in the upper part for a 
certain distance from the fixed ends, but for 



the purpose of this present explanation it will 
be considered that the beam is supported 
only. 

There are several formulae for the moment 
of resistance of a reinforced concrete beam, 
but it is not sufficient to give these without 
explaining the principle of their construction. 

It is shown in the preceding chapter that 
the compressional and tensional resistances 
must be equal to one another, and that they 
are acting in opposite directions and are 
parallel, thus forming a " couple," and the 
value of the moment of resistance of the 
whole section is the amount of one resisting 
power multiplied by its distance from the 
other. 

The resistance to compression is supplied 
by the concrete above the neutral axis, while 
the resistance to tension is supplied by the 
steel in the bottom of the beam, the concrete 
below the neutral axis being neglected, as 
shown in Figs. 117 and 118. Here it will be 
seen that the stress on the concrete is shown 
as being of a uniform intensity acting over 
an equivalent inertia area which, as pre- 
viously explained, is equal to a varying 
intensity over the whole of the compressional 
area. Hence the equivalent compressional 

area will equal -^-, where 6 = the breadth 

of the beam in inches and n = the distance 
of the neutral axis from the compressed edge. 
The safe compressional stress per square inch 
on the concrete is expressed by c, and the 
value of the compressional resistance, there- 

C t) YL 

fore = . The safe resistance of the 

A 

steel in tension per square inch is expressed 
by the symbol t, and the sectional area in 
square inches by At, therefore the value of 
the tensional resistance equals At x t. 
The total compression and total tension 

are equal, and therefore ~- = t At. These 

two equal resistances or internal forces act 
in opposite directions, and the amount of 
one of them must be multiplied by the dis- 
tance between them to find the moment of 
resistance. The centre of gravity of the 
compressional area, at which point the com- 
pressional resistance can be considered as 
being concentrated, will be situated at a 

distance of from the top edge, or at a 
o 

distance of f n from the neutral axis. The 
tensional resistance will be acting at a 



86 



REINFORCED CONCRETE 



point equal to d n from the neutral axis 

*i 

or d -x from the centre of gravity of the 

compressional area. Therefore the moment 
c b n 



of resistance = 



('-8 



or t At x 



d , and this must be equal to the 

c b n 



( 

bending moment, B M. Thus B M = ^ 

f n\ f n\ 

x [ d o I or B M = Z A ( x I I 

V >/ v */ 



Before any use can be made of these 
formulae it will be necessary to establish some 
relation between certain factors which are 
variable and unknown, namely, c, t, and n. 

The factors 6, d, and A t can be settled by 

b 



'i AX 15 




will require fifteen times as much stress on 
the steel as that on the concrete to produce 
this result if the two materials are situated 
at the same distance from the 'neutral axis. 
This is illustrated in Fig. 119, where it is 
assumed for the moment that the neutral 
axis is situated at the centre of the depth ; 
then the permissible stress on the concrete 
is 600 Ib. per sq. in., and the stress on the 
steel will equal c x m, which equals 600 x 15 
= 9,000 Ib., m being the ratio between 
E s and E c . 

It must be clearly understood that 
although the deformation does not actually 
take place if the beam is properly designed, 
there is a tendency for it to do so, and conse- 
quently the resistance has to be exerted by 
the materials to overcome the tendency. 



CEMTRE OP 
COMPEt55IONAt 




Fig. 117 Fig. 118 

Figs. 117 and 118. How a Single Reinforced Beam Resists Compression and Tension 



the designer himself, but the value of n, t, 
and c will be affected by the relation or pro- 
portion of At to 6 d, and as there are two 
unknown factors in each formula it would not 
be possible to proceed beyond a certain point. 
Again, it will not be possible to stress both 
the steel and the concrete to the permissible 
limit unless they are in the correct proportion 
to one another. The two materials have a 
different coefficient of elasticity which in 
the concrete is expressed by E c . and in the 
steel by E s , and the ratio between them = 
E, 



E.. 



= 15, and this is an important point in 



the determination of the stresses. It signifies 
that in the case of deformation taking place 
where the concrete is compressed by a cer- 
tain stated amount (say, 1 in.), while the 
steel is stretched by the same amount, it 



The permissible stress on the steel is 16,000 
Ib., and therefore if the proportion between 
the two materials is such that the neutral 
axis is at the centre of the depth, the steel 
cannot be used to its fullest capacity. It 
will be necessary to have the proportion 
such that the neutral axis is higher up the 
beam, and thus farther from the steel, be- 
cause the stress varies directly as the dis- 
tance from the neutral axis, and thus, in 
addition to the ratio m, it is necessary to 
consider the relative distances n and d n. 
If the steel is 2 in. from the neutral axis 
while the concrete is only 1 in. from the 
same point, then the stress on the steel will 
be equal to twice that in the concrete ; or, 
in other words, t = c x 2. The same rule 
will apply if it is said that the stress in the 
steel multiplied by n will equal the stress 



THEORY OF REINFORCED CONCRETE 



in the concrete multiplied by d n, or 
I x n = c x d n. Now taking into con- 
sideration the ratio m and the distance from 
the neutral axis, the result is t n = m c (d n). 
It is allowable to reduce this to give a 
relative proportion for n as follows : 

in = m c (d n) 
tn = mcd men 
tn + men = mcd 
n(t + me) = mcd 

mcd n me 






t + me 



"" 



t + me 



In the case of this formula there are cer- 
tain factors known if c and t are considered 
as representing the permissible stress on the 
concrete and steel ; thus, m =15, c =600 Ib., 



AX 15 




Fig. 119. Diagram Showing Proportionate 
Stresses above and below Neutral Axis to 
Produce Deformation 

and t = 16,000 Ib. With the aid of these it 
is possible to deduce n as a definite propor- 
tion of d, as follows : 

mcd 

n = 

t + me 

15 x 600 Ib. x d 
n = rt 



n = 



16,000 Ib. + (15 x 600 Ib.) 

9,000 d 

25,000 



n = -~~ d = n -36 d. 

Now this is of great value, as it shows that 
in the case of a well-designed beam where 
t and c are at the permissible limit, then n 
= -36 d. It does not necessarily follow that 
c and t will be equal to the values given 
above, as it has previously been stated that 



this will only occur when A t and 6 d, the areas 
of the steel and concrete, bear a certain pro- 
portion to one another. 

The next step, then, is to determine this 
proportion, and see how the neutral axis will 
vary if another proportion is employed. 

The ratio of the area of the tensile rein- 
forcement to the area b d is expressed by the 

symbol r, therefore r = r-v 

Now it has already been seen that t A t 

cbn cbn A t 

= -s , so that At = - . If r = j-j 

2 ' 2t bd 

cbn cbn 

then r also equals . , , . as -^rr is the 
Alba At 

equivalent of A< and can be substituted for 

cbn 
it. In the expression r = 0.77, the 

symbol b can be eliminated, as it occurs in 

c n 
both cases, thus reducing it to r = ^~TJ- 

An expression is now obtained which will 
give the ratio of the steel to the concrete to 
develop certain given values for c and t. 
In the first place, consider the case where 
c = 600 Ib. and t = 16,000 Ib. as above, 
where it was seen that n = -36 d, then c, n, 
t, and d are known, thus 

600 x -36 



r = 



T = 



r - 



2 x 16,000 x 1 
216 

32,000 

27 

= - 00675 - 



It will be seen by this that if the greatest 
permissible stress is represented by c = 600 
Ib. and t = 16,000 Ib., then n = -36 d and 
r = -00675 or A t = -00675 b d. 

Applying the Formulae. Befoie pro- 
ceeding to the derivation of any more 
formulae, it will be as well to apply those 
obtained up to the present and become 
more or less familiar with their use, to pre- 
vent the subject from becoming too com- 
plicated for the student who is dealing with 
symbols and reasoning which are quite new 
to him. 

Briefly summarised, the following have 
been deduced : (1) The total compression 

cbn 
and total tension are equal, expressed ^~ 

= t A t . (2) The moment of resistance 
equals either of these multiplied by d - -, 
which is the lever arm of the internal forces, 



88 



C u 71 

expressed M R = x 



REINFORCED CONCRETE 



- ^ ) , or M R 



= Z A t x ( d - J . (3) These must be equal 
to the bending moment, expressed B M = 
'-. ) or BM tA t x 



At = -00675 x 8 x 12-5 

At = -00675 x 100 

At = -675 sq. in. 

The moment of resistance can now be cal- 
culated according to formulae in deduction 
No. 2, namely : 

cbn / _ n> 
MR = - 



( d - ! j. (4) The position of the neutral MR = 



600 x 8 x 4-5 



axis is dependent on the coefficients of elas- 
ticity, m = 15, and the values of c and t, 

expressed n = 7 , where c = 600 lb. 

t "T" 7/1 C 

and t = 16,000 lb., then n = -36 d. (5) 
The ratio of the steel reinforcement to the 
area of the beam will depend on the values 

c n 
of c and t, and will be expressed r = 2~Td' 

when c = 600 lb. and t = 16,000 lb. ; 
then r = -00675 or At = -00675 b d. 

Calculating Amount of Reinforce- 
ments. As a first example, assume a con- 
crete beam 12^ in. deep and 8 in. wide. It is 
required to find the amount of steel neces- 
sary to reinforce the beam if the working 
stresses are to be c = 600 lb. and t = 16,000 
lb. per sq. in. Also calculate the moment 
of resistance of the beam when so reinforced, 
and find what uniformly distributed load it 
will carry over a span of 15 ft. if the ends 
are supported only. 

The values of c and t being known, the 
first step is to ascertain the position of the 
neutral axis. By using the formula in 
deduction No. 4, it will be found n = -36 d. 
Therefore n = -36 x 12-5 

n = 4-5 in. 
Work this out by means of the formula 

mcd . . . 

n = - for the purpose 01 getting 

t }- m c 

quite familiar with the use of same, then n = 

15 x 600 x 12-5 112,500 

16,000 + 15 x 600 '' 25,000 " 

in., as before. 

Having derived the value of n, the amount 

C YL 

of steel willjbe given by r = ^j, and this 

should work out at -00675, as c = 600 lb. 
and t = 16,000 lb. 

600 x 4-5 

= 2 x 16,000 x 12-5 
2,700 
400,000 
r = -00675 
At therefore equals -00675 b d. 



I 4-5\ 

( 12 - 5 -IT) 



~ 



M R = - x (12-5 - 1-5) 

MR = 10,800 x 11 
M R = 118,800 in.-lb. 
Again, working the moment of resistance 
according to the tensile reinforcement, 

M R = t At x ( d - 

/ 4-J 

MR = 16,000 x -675 x ( 12-5 - -j 

MR = 16,000 x -675 x (12-5 -1-5) 

MR = 16,000 x -675 x 11 

MR = 10,800 x 11 

M R = 118,800 in.-lb., as before. 

As the moment of resistance equals 118,800 
in.-lb., the bending moment must not 
exceed this amount, and the next step is to 
ascertain what weight would produce this 
bending moment. 

In the case of a uniformly distributed load 

with supported ends B M = -g-, therefore 

118,800 in.-lb. = -g-. w is the unknown 

quantity, and I = the span in inches = 
15 x 12 = 180. 

118,800 in.-lb. = 



118,800 x 8 



w " 

w = 5,280 lb. 

w = 47-14 cwt. 



The beam will therefore require -675 sq. in. 
of steel reinforcement, and safely carry a 
load of 47-14 cwt. over a span of 15 ft. 

Calculating Beam for Certain Load. 
In this example, the size of the beam was 
given and the reinforcement and the load 
were calculated, for the sake of applying 
the deductions previously made. In actual 
practice it is usually required to design a 
beam to carry a certain load or to check 
a given beam and ascertain if the safe 
stresses on the concrete and steel have been 
exceeded. As an example of the latter 



THEORY OF REINFORCED CONCRETE 



89 



instance, the beam just dealt with can be 
calculated, when the stresses should agree 
with those arranged in the question, namely 
c = 600 Ib. and t = 16,000 Ib. The bend- 
ing moment to be dealt with is 118,000 
in.-lb., and as shown in deduction No. 3, 



BM = -o x 



/ W\ 

( d ~~3J- 



The value of c will 



therefore be found by the formula c = 
2BM 



2 x 118,800 



x I a 



c = 



8 x 4-5 x 
237,600 



396 



= c = 600 Ib. 



This amount is correct, and the stress in the 
steel can next be calculated as follows : 



BM t A t xid ^ J , therefore 
BM 



t = 



A, 



t = 



118,800 



t = 



118,800 



7.4.25 



_ 

t 16,000 Ib. 



This, again, agrees with the stress allowed, 
and thus the two formulae are obtained 
which will give the stresses in the beam 
when this has to be checked after it has 
been designed. 

In the actual designing of the beam, the 
load will be a definite quantity according 
to circumstances, and the bending moment 
will therefore require to be calculated in the 
first instance. It will be necessary, there- 
fore, to have some equation which will give 
a suitable size beam to resist such bending 
moment, and also some method of obtain- 
ing the position of the neutral axis when 
the values of c and t are unknown, such as 
would be the case in designing to resist a 
given bending moment. 

The next step is to establish some relation 
between 6, d, and the bending moment. 
The draft regulations of the London County 
Council limit the minimum depth of a beam 
to one-twenty-fourth of the span, but as the 
compressional resistance depends on the area 
of concrete above the neutral axis and the 
resistance must be equal to the bending 



moment, it is preferable to calculate from 
the bending moment direct without regard 
to the span. Referring to deduction No. 3, 

begin again with the equation B M = ~ x 



( d - |Y 



Thus 2 B M =c x b x n x 



(d - | 



The value of c can be taken at 600 Ib., and n 
should equal -36 d if the economical per- 
centage of reinforcement is to be employed, 
this leaving only two unknown factors b and 
d. If b is settled as some definite proportion 
of d, then this proportion can be substituted 
and d expressed as a fraction of B M. A 
well-proportioned beam is frequently that in 
which b = -6 d, and this will be taken as the 
value, hence 



2BM = c x & x n x [d 7; 



2 B M = 603 x -6 d x -36 d x (d- 



2 B M = 600 x -6 d x -36 d x (d 12 d) 
2 B M = 600 x -6 d x -36 d x -88 d 
2 B M = 600 x -19008 d 3 
2 B M = 114-048 d 3 
B M = 57-024 d 3 
BM 



57-024 



d 



3 / 

3 V 



BM 

57-024 



To simplify matters and save a great 
number of figures in the calculations, it will 

be sufficient to use d = ?/ , the dele- 

V 57 

tion of the decimal figures in this case making 
so little difference that it is inappreciable. 
The writer is aware that there are many 
students who have forgotten the method of 
finding the cube root of a number, but when 
this is the case the student should use a 
table and thus overcome any difficulty on 
this point until such time as he shall again 
become conversant with the method. In 
the absence of a table, the cube root can be 
found by trial, but, needless to mention, 
this is somewhat of a tedious method. 

To illustrate the application of the formula, 
it will be advisable to work out an example, 
as before, the load and span being given. 
It is required to design a beam supported at 
the ends to carry a load of 650 Ib. per foot 
run over an effective span of 15 ft. It is 
assumed that this load includes the weight 
of the beam itself, and the safe stresses are 
to be 600 Ib. and 16,000 Ib. 



9 o 



REINFORCED CONCRETE 



The total load = 650 Ib. x 15 = 9,750 Ib. 

id 9750 x 180 
Ihe bending moment = -Q- = - ~ 

B M = 219,375 in.-lb. 

3 A 
Therefore d = //- 



'219375 



_57 

d = ^3848 

d = 15-7 in. 
The breadth should be -6 d, therefore 

b = -6 x 15-7 in. 

b = 9-4 in. 

The size of the concrete beam itself has 
therefore been found, and the amount of 
reinforcement is now required. 

According to deduction No. 5, the ratio of 
steel to concrete should be -00675. Therefore 
At = -00675 b d, b d = 147-58 sq. in., and 
At = -00675 x 147-58. A t = -996 sq. in. 
say three y|-in. diameter round bars. 
This will give the nearest area, always bear- 
ing in mind that there should be an excess 
rather than a deficiency. The area of one 
y-in. diameter bar = -3712 sq. in., and the 
total area of the three bars = 1-1136 sq. in. 
The beam as designed has an effective 
depth of 15-7 in. To this must be added, 
say, 2 in. of concrete to cover the bars, 
giving a total depth of 17-7 in. The com- 
pleted section should therefore be : depth, 
17f in. ; breadth, 9| in. ; reinforcement, 
No. 3 y-in. diameter round rods. 

Checking the Beam. In the case of 
beams which have to be checked for the 
purpose of ascertaining the values of c and 
t, the necessary formulae up to a certain 
point are at hand, but some further ex- 
planations and deductions are required to 
make the data complete. As previously 
shown, the stress in the concrete can be 

2BM 

found by the formula c = - r > 

-*M) 

and the stress in the steel can be ascertained 
by t = - ,- The difficulty in 



the case of these formulae will be that of 
finding the value of n. Previously, the 
position of n has been said to be dependent 
on the coefficients of elasticity and the 
values of c and t. 

In the case of checking a beam already 
designed c and t are unknown, and the 
formula cannot be used, and if the ratio of 
steel to concrete is not -00675, then n will 



not equal -36 d. The position of the neutral 
axis, being dependent upon the comparative 
values of c and t, must also be dependent 
upon the proportion of steel to concrete, 
and as both of these will be given in the 
beam as designed, a value for n must be 
deduced accordingly. 

It has already been shown that t n = 

, . . , t m x (d n) 
cm (a n), and therefore - = -. 



cbn 
Again, 2 

9 t A 



= t A t , and therefore n = 



Now t and c are unknown, in which 

C 

case substitute the value for -, namely, 



m (d n) 



Then n = 



2 A t m (d n) 



and 



b n 

bn 2 = 2 A t m (d n). Here n is on both 
sides of the equation, and it must be reduced 
as follows : 

b n 2 = 2 At m (d n) 
bn 2 = 2 Atmd 2 A t mn 
b n 2 + 2Atmn=2Atmd 
Divide both sides by 6, then 

2 A t mn 2 A t md 
^ + 1 T- 



Add 
then 



(A, wiV 
^-) 



to both sides of the equation, 



2 A t m n 

~b ' 



A t m\ 2 2 A t m d 



n + 
Therefore 



A, my _ 

b I 

At m 



(A t m 
\ *> 



+1- 



+ 



/2 

v 



~A t md A t m 



2A t md 



+ 



(A 1 m\ a 



m 



Finding Position of Neutral Axis. 

A formula is now to hand for finding the 
position of the neutral axis in the case of a 
beam which has been designed, where the 
stresses may or may not be in accordance 
with the permissible limits. 

Assume, as a first example, that the beam 
given in Fig. 120 has been designed to resist 
a bending moment of 219,375 in.-lb., and it 
is required to ascertain if the stress in the 
concrete and steel is in accordance with the 
values allowed, namely 600 Ib. and 16,000 Ib. 
This is actually the same beam that was 
calculated in the previous example, and it 
should be noted that the area of the steel 



THEORY OF REINFORCED CONCRETE 



reinforcement required was only -996 sq. in. ; 
but it was increased to 1-1136 to make it 
possible to use rods of a practical size, and 
it is for the latter area that it must now be 
calculated. The sizes of the concrete are 
also the practical sizes, such as would appear 
on a drawing of the beam, when the theo- 
retical sizes would not be available to anyone 
checking the stresses in the ordinary way. 
The first step is to ascertain the position 



n 



R 



Fig. 120. Finding Position of Neutral Axis 

of the neutral axis, and this will be done by 
means of the formula given above, namely : 



= V 

V- 



2 A t md 



V b ) 



2 x 1-1136 x 15 x 15-75 
9-5 

1-1136 x 15 
9-5 



/l-1136xl5V 
V 9-5 ) 



n = 



526-176 



16-704\ 2 16-704 



9-5 '_ \ .9-5 / 9-5 

n = N /55-386 + (1-758) 2 - 1-758 
= v/58-476|-l-758 
n = 7-64 - 1-758 
n = 5-882 in. 

It will be seen that this result differs 
slightly from that which would have been 
obtained by taking n = -36 d, as in the 
latter case this equals -36 x 15-75 = 5-67 in., 
and this difference has arisen on account of 
A t and b d having been altered to give prac- 
tical sizes. Having now obtained the value 



of n, the values of c and t can be found as 
follows : 

2BM 

c = - 

b n : 



c = 



2 x 219375 



c = 



9-5 x 5-882 x 

438750 

9-5 x 5-882 x 
438750 



13-70 



A< x ( d - o 



~ 770-57 
c = 569 Ib. 

This is below the permissible stress of 600 Ib. 
per sq. in., and the value of t may now be 
determined : 

BM 

t = - 



t = 

t = 

t = 
t 



219375 



1-1136 

219375 



"(' 



5-882 



1-1136 x 
219375 
15-356 
14285 Ib. 



13-79 



This value, again, is well below the per- 
missible stress, and consequently the beam is 
quite satisfactory for the load it has to carry. 

It is interesting to note that the area of 
steel required theoretically was increased by 
about 12 per cent, to give a practical area, 
and this -reduced the stress on the steel per 
square inch by about 10 per cent., and the 
stress on the concrete by about 5 per cent. 

In the draft regulations of the London 
County Council, it will be seen that the 
formula for finding the position of the 
neutral axis is given as : 

n = [ v ''m 2 r 2 + 2 m r) mr]d 
and a note in explanation of this should be 
of interest to those who are not sufficiently 
advanced to trace the construction and 
evolution of the formula. The symbol r, as 
previously stated, equals the ratio of steel to 
concrete, and the formula is constructed to 
use this ratio for finding the position of n 
when c and t are unknown. Upon reference 
to the reasoning employed in deducing the 
previous formula for the position of n, it will 
be seen that b n z = 2A t m(d n). Therefore 

2 A t m (d n) 

n 2 = - r- 



REINFORCED CONCRETE 



Now in this case A t is also unknown, and is 
simply available as a proportion of the con- 
crete, such proportion being expressed as r. 
We can therefore substitute this equivalent 
and obtain 

2 _2rbdm(d n) 

Hence 

n 2 = 2rd 2 m2rdmn 
n 2 + 2r dmn = 2r d 2 m 

To both sides of the equation add (r d m) 2 . 

Then 

n 2 + 2rdmn + (r d m) 2 = (r d m) 2 + 

2rd 2 m. 

The left-hand side of the equation can 

now be simplified as before, because n + 

r d m is the square root of same, and therefore 

(n + r dm) 2 can be substituted as its value, 

giving (n +rdm) 2 = (rdm) 2 + 2rd 2 m. 

Hence 

n+rdm=*J(rd m) 2 +2rd 2 m 
n = \/ (r d m) 2 + 2rd 2 m rdm 
n =[v/( r m ) 2 + 2 r m r m] d 
This, of course, is the same as the formula 
given by the London County Council, 
namely : 

n = [v/wi 2 r z +2mr mr\d 
In the previous notes it was shown that 
when r = -00675, which is the economical 
proportion of steel to concrete, then n 
would always equal -36 d. It will be advis- 
able, therefore, to use the above formula and 
find what proportion n will be of d if the 
economical amount of steel is used as an 
illustration of the use of the formula, as 
follows : 

n = [v/wi 2 r 2 +2mr mr]d 

m 15, and r = -00675, therefore 

n = [v/15 2 x -00675 2 + 2 x 15 x -00675 

- 15 x -00675] d 

n = [^225 x -0000455625 +-2025 10125] d 
n = [x/ -0102515625 + -2025 - -10125J d 

V/-2127515625 - -10125] d 
n = -46125 - -10125]<2 
n = -36 d 

A great number of figures are retained 
after the decimal point with the object of 
showing that the result absolutely agreed 
with that previously given. Formulae, there- 
fore, are available for finding the value of 
n in all cases, namely (1) when c and t only 
are known, (2) where the size of the beam 
and area of reinforcement are known and c 
and t are unknown, and (3) when c and t are 



unknown and the area of the steel is expressed 
as a proportion of the area of concrete. 

Designing Beam for Certain Condi- 
tions. As a final example, showing the 
application of the various formulae, a case 
will be taken where it is required to design 
a beam to carry a uniformly distributed 
load of 350 Ib. per foot run, in addition 
to its own weight, over a span of 24 ft. 
When designed the stresses in the steel and 
concrete will be checked to ascertain whether 
they are in accordance with the permissible 
limits. 

It will here be seen that the weight of the 
beam itself has to be added to the external 
load before the bending moment can be 
found, and as the size is not yet known, an 
assumption must be made in order to do 
this. A little experience will enable the 
designer to gauge the size sufficiently near 
the actual size required to give a weight 
for the beam. In this instance the depth will 
be taken at 16 in., which is one eighteenth 
of the span, and b can be taken at 10 in., 
which is just over -6 d. To the amount of 
d 2 in. should be added as a covering for 
the reinforcement, giving a total size of 
18 in. by 10 in. The weight of the beam 
will be its cubical contents multiplied by 
150 Ib. per cub. ft. This equals 24 ft. x 
1-5 ft. x -63 ft. 'X 150 Ib., or for 1-ft. run 
= 1-5 ft. x -63 ft. x 150 Ib. = 141-75 Ib., 
say 142 Ib. 

The total load to be carried per foot run 
therefore equals 350 + 142 = 492 Ib. This, 
multiplied by the span, will give the total 
load on the beam, thus 492 x 24 = 11,808 

Ib. The bending moment will equal -^-. 



BM = 



11808 x 24 x 12 



B M = 425,088 in.-lb. 
The moment of resistance of the beam must 
equal 425,088 in.-lb., and the depth must first 

3/ 

be calculated by the formula d = A/ 



BM 

57 ' 



d = 



425088 



d = ^/7457 
d = 19-5 in. 

Take 6 at -6 d = 19-5 x -6 = 11-7 in. The 
area of steel will be given by taking the 
economical ratio, namely -00675. 
A t = -00675 b d 
At = -00675 x 11-7 x 19-5 
At = 1-54 sq. in. 



THEORY OF REINFORCED CONCRETE 



93 



If three bars, |f in. in diameter, are taken, 
this will give an area of 1-55 sq. in., which 
will be quite suitable. The beam, as now 
designed, has an effective depth of 19-5 in., 
with a breadth of 11-7 in. and steel reinforce- 
ment of 1-55 sq. in. Add 2 in. to the depth 
for covering the reinforcement, and make 
the breadth a practical size, and the beam 
becomes say 22 in. by 12 in. 

Checking Beam after Designing. It 
is now required to check the stresses in this 
beam, and as the actual size of the beam is 
now available it will be advisable to work 
out the bending moment again, because the 
calculated size is in excess of that assumed 
in the first instance. 

Therefore the weight of the beam per foot 
run - If ft. x 1 ft. x 150 Ib. = 275 Ib. 
Total load per foot run = 350 + 275 = 625 
Ib. The total load to be carried equals the 
load per foot run multiplied by the span = 
625 x 24 = 15,000 Ib. The bending moment 
wl , _, 15000x24x12 

= -A-. Therefore B M = ^ 

o o 

= 540,000 in.-lb. 

The position of the neutral axis must next 
be found by the formula 

/2 A t m d /A t m\ 2 A t m 

/2 x 1-55 x 15 x 19-5 /1-55 x 15\ 2 
n =V ~W~ H 12 / 

1'55 x lo 
12 



t = 



BM 



t = 



540000 



1-55 



t = 



t = 



540000 



1-55 x 17-18 
540000 



m.-lb. 



/906-75 /23-25X 2 
= V 12 ' \~12~) 



23-25 



12 \ 12 / 12 

n == V75-562 + 3-751 - 1-93 
n = v/79-313 - 1-93 
n = 8-9 - 1-93 
n = 6-97 in. 

The stress in the concrete can now be 
found by the formula 

2BM 

c = 

b n > 



c = 



c = 



2 x 540000 



12 x 6-97 x 

1080000 
12 x 6-97 x 17-18 



1080000 
= := c ~-~- 7531b " 



This is too high, as the permissible limit 
is 600 Ib. 
Now check the stress in the steel. 



This, again, is much too high, as the per- 
missible limit is 16,000 Ib. for ordinary mild 
steel, and even for stronger steel the stress 
should not exceed 20,000 Ib. 

This example was given to show the value 
of checking the stresses after the beam has. 
been designed, when the size has to be 
assumed, and consequently an error of judg- 
ment may take place, especially in the case 
of those inexperienced in design. 

The next step is to see in what manner the 
beam can be adjusted to bring the stresses 
within the limits. The weight of the beam 
allowed in the first instance was insufficient, 
it being only 142 Ib. per foot run, whereas 
the calculated beam was 275 Ib. per foot run. 
Here, however, is some basis upon which to 
assume the size, and if the depth is increased 
to 22 in., and the breadth to -6 d = say 
13-5 in., they cannot fail to be near the 
required size. 

This may seem a great deal of work and 
calculation in connection with one beam, 
but it must be borne in mind that the 
example is merely given to illustrate princi- 
ples and familiarise the reader with the 
formulae employed. 

Again, in a building where the plan is 
symmetrical, it usually happens that there 
are several beams which have the same span 
and carry the same amount of load, and con- 
sequently when the calculations for one beam 
are complete, many beams can be considered 
as designed, and it is worth some little 
trouble to produce economical and efficient 
design. 

The reinforcement might be increased in 
this example, and thus the strength of the 
beam increased, but the economical per- 
centage would not then be employed. 

Of course, it is quite simple to design a 
member which is perfect and suitable for 
carrying a certain load as given in various 
textbooks, but a great number of these 
examples are done by selecting a beam with 
a certain ratio of reinforcement, finding the 



94 



REINFORCED CONCRETE 



moment of resistance, and working back- 
wards, as it were, to find the safe load per 
foot run over a certain span. This method, 
of course, is not stated, but the examples 
frequently given in the textbooks are based 
on the particulars obtained, and thus no 
difficulty is encountered. 

The object of the above example is rather 
to show how to deal with a case where the 
initial calculation is found to be incorrect, 
and to show in what manner these may be 
utilised in the second calculation. It will 
be seen that in the first instance the depth 
was based on the span, as the bending 
moment was unknown, and it goes to show 
how essential it is to have some connection 
between the depth and the bending moment 
if economical design is to result. However, 
the actual bending moment to be resisted is 
known when the beam had an effective depth 
of 19-5 in. and a breadth of 12 in., namely, 
540,000 in.-lb. Calculate d according to 



this as a guide by the formula d = 



BM 

57 ' 



then d = 



540000 
57 



= d = y 9473, d = 



21 in. nearly. It must be realised, however, 
that the fact of increasing the size of the 
beam will again increase the weight of the 
beam, and therefore the bending moment, 
and an allowance must be made for this ; 
and for this reason it will be advisable to 
take d at 22 in. as stated above, and b at 
6 d = say 13-5 in. 

The beam can now be worked out and 
again checked to see if the stresses are quite 
satisfactory. The size of the beam = 22 in. 
x 13-5 in., and the sectional area = 297 
sq. in. A t should equal -00675 b d. This 
equals 297 x -00675 = 2-00475 sq. in. 
The area of a round rod with a diameter 
of || in. = -6903 sq. in., and 3 rods = 
2-0709 in., which is more than sufficient. 
Provision must be made for covering the 
bars with concrete, and if 2 in. be given, the 
total depth becomes 22 +2 = 24 in. 
Having now designed the beam, the actual 
stresses can be checked, and to do this the 
actual bending moment must be found due 
to the given load to be carried, and the 
weight of the beam itself. 

The latter per foot run = 2 x 1 x 150 Ib. 
= say 337 Ib. The total load per foot run 
= 350 + 337 = 687 Ib. The total load = 
load per foot run x span = 687 Ib. x 24 



= 16,488 Ib. The bending moment = 

Mel 16488 x 24 x 12 

r, therefore B M = ~ 

= 593,568 in.-lb. 

The position of the neutral axis must next 
be found by the formula 



2A t md 



/ t 
n =\/b + 



Lrw\ 2 _ A t m 
b ) b 





/2 x 2-0709 x 15 


x22^ 


C 1 


0709 


x!5> 


2 




/ 13-5 

2-0709 x 


15 


( 


13- 


5 ; 


13-5 





/ 1366-794 /31-0635\ 2 31-0635 

ss / . _ _1_ I _ I _ _ 

V 13-5 ^V 13-5 / 13-5 



n = v/ 101-24 +5-294 - 2-301 
n = 10-32 - 2-301 =n = 8-01 in. 

The stress in the concrete can now be 
found by the formula 

2BM 



b n x 



(-9 

2 x 593568 



c 



13-5x8-01 



1187136 



c 



13-5 x 8-01 x 19-33 
1187136 



2090-2 
c = 567-9 Ib. 

It will be advisable to check the stress in 
the steel as follows : 

BM 



t = 



A 1 x 



t = 



('-8 

593568 



2-0709 x 



t = 



t = 



593568 

2-0709 x 19-33 
593568 



40-03 
t = 14,828 Ib. 

This amount is well below the permissible 
limit of 16,000 Ib. The beam as designed 
will therefore have a total depth of 24 in., 
a breadth of 14 in., and be reinforced with 
three -J-g--in. diameter rods. 



THEORY OF REINFORCED CONCRETE 



95 



Numerous problems and examples should 
be worked out with a view to becoming quite 
proficient in the use of the formulae, and 
conversant with the method of making 
assumptions which are necessary" in some 
cases to form a basis for the calculations, 
since, although the principles never vary, 
the method of procedure is likely to do so 
under different circumstances. 

The beam would also be required to be 
calculated for shear and adhesion, but these 
will be dealt with subsequently. In any 
examples shown up to the present any 
stress other than that due to simple bending 
has been disregarded. 

CALCULATIONS FOR SLABS 

Slabs can be dealt with in a similar manner 
to beams as regards the calculations of 
strength, but the value of the bending 
moment will vary according to the propor- 
tion of the slab, if the slab is supported on 
all four sides ; the formula for finding the 
depth in relation to the bending moment 
must be varied, as b will no longer be equal 
to -6 d, but will always equal 12 in., as a 
strip of this width is calculated. In the case 
of a slab supported on all four sides and 
reinforced in two directions, the weight will 
be distributed on all four supporting edges, 
and consequently if a strip of the slab 12 in. 
wide is taken and the bending moment cal- 
culated, such bending moment can only be 
due to the reactions which occur at the ends 
of the strip. If the total weight upon the 
slab is distributed upon all four edges, it 
will follow that the reaction at any given 
point will be less than if the slab were sup- 
ported on two sides only. A reduction in 
the bending moment must therefore be 
made to allow for this. If the slab is per- 
fectly square, all four edges will carry an 
equal amount, and the reaction at any one 
side will be one-half the amount that would 
occur if the slab were supported on two sides 
only. If the reaction is only half the 
amount, then the bending moment will be 
reduced by a similar factor. 

The bending moment at the centre of a 
beam or slab supported at the ends is equal 

w I 
to --, and therefore the bending moment 

at the centre of a square slab, supported on 

wl 

all four edges, is equal to -TTT. Again, the 

bending moment at the centre of a beam or 
slab which is fixed at the ends is equal to 



-, - 



, while with a square slab fixed on all four 



.TIT W 

edges, the bending moment is equal to -^ 

These formulae, of course, apply to cases 
when the load is uniformly distributed. 
This advantage may be considered as occur- 
ring when the length of the slab does not ex- 
ceed twice the breadth ; when the length 
exceeds this proportion, the slab is considered 
as one which is supported or fixed on two 
edges only. The advantages will vary be- 
tween these two cases, namely, a square 
slab, and a slab where the length is twice 
the breadth, and the variation will depend 
upon the ratio of the length to breadth. 

The draft regulations of the London County 
Council give formulae showing the allowance 
that may be made as follows : 

w = weight on slab (total distributed 
weight, including its own weight). 

b = breadth of slab. 

I = length of slab. 



Bending 


Condi- 


B.M. 


B.M. 


moment at any 


tion 


for 


for 


given 


of 


shorter 


longer 


cross section 


supports 


span 


span 






wb 1 


wl 1 


At centre of span 


Free 






8 l + (b/l)* 


8 l + (l/b)* 


At end and 
centre of span 


Par- 
tially 
fixed 


wb 1 


wl 1 


12 l + (6/J) 4 


12 1 + (*/&*) 



Bending Moment at Centre of Slab. 

It will be advisable to give an example to 
illustrate the use of these formulae and show 
the decrease in the bending moment which 
is given by same. Assume a slab 8 ft. by 
6 ft., which is supported on all four sides, 
and carries a uniformly distributed load of 
250 Ib. per sq. ft., and let it be required to 
calculate the bending moment at the centre 
of the span in both directions, namely, (1) 
by considering a portion 1 ft. wide and 
6 ft. long, and (2) by considering a portion 
1 ft. wide and 8 ft. long. 

By referring to the table above, it will be 
seen that the formula for the shorter span 

is X w = the total 



and this will equal the area x by the weight 
per ft. super. Therefore w = 6 ft. x 1 ft. x 
250 Ib. ; w = 1,500 Ib. ; 6 = the breadth 



9 6 



REINFORCED CONCRETE 



of the slab, which is 6 ft. ; I = the length 
of the slab, which is 8 ft. Hence, 
t0& ^L 
8 X 1- 



1500 Ib. x 72 in. v 1 

; . X 



1 



= 13500 x 



= 10,258 in.-lb. 



1-316 

The allowance made in this instance is 
sufficient to reduce the bending moment 
from 13,500 in.-lb. to 10,258 in.-lb. 

Now calculate the bending moment for 
the longer span, according to the formula 
wl 1 



In this case w = 8 ft. x 1 ft. x 250 = 
2,000 Ib. Hence, B M = 

O i 



B M = 24000 x 



1 



- 24000 x 



4-16 



1 + 3-16 
B M = 5,769 in.-lb. 

The reduction is considerably more in this 
instance, due, of course, to the fact that 
the edges of the short span will carry a 
greater portion of the load than the 
edges of the long span, and as the reactions 
will be less, so the bending moment will 
be reduced. 

With regard to the thickness of the slab, 
this should be calculated according to the 
bending moment, and the following formula, 
with its construction, is given for this pur- 
pose. It has already been seen by previous 

(n\ 
d ~~ Q )' 
./ 
and that it is possible to substitute definite 

values for certain symbols as follows : b 
= 12 in., c = 600 Ib., and n = -36 d, thus 
reducing the formula to 

2 B M = 12 x 600 x -36 d x (d - 



2 B M = 12 x 600 x -36 d x -88 d 

2 B M = 7200 x -3168 d 2 

2 B M = 2280-96 d 2 = B M = 1140-48 d 2 



/ BM 

= V 1140-' 



M 

1140-48 
It will be quite near enough to use the 

/B~M 
lormula as a == /y/ VMA' e omission of 

the decimal figures making no appreciable 
difference in the result. In the case of a slab 
which is supported on all four edges, it has 
been shown that it is necessary to calculate 
the bending moment for both spans, and as 
the shorter span will give the greater bending 



moment, as shown by the example above, the 
depth should be calculated to suit this, and 
as this will be in excess of that required for 
the longer span, the longitudinal reinforce- 
ment required will be diminished accord- 
ingly. Again, it will obviously be an advan- 
tage, theoretically, to place the reinforce- 
ment across the slab below that parallel 
with the length of the slab, in order to obtain 
the maximum effective depth to resist the 
greater bending moment. 

If the depth is found by calculation to be 
less than 3| in., it should be increased to this 
amount for practical purposes, as any thick- 
ness under this is likely to be unreliable. 
The least diameter or thickness of the main 
reinforcement should not be less than \ in., 
and any other reinforcement should not be 
less than \ in. in diameter. The maximum 
distance between the main tensile reinforce- 
ment should not be greater than 12 in., or 
less than 1 in., and the spacing of the rein- 
forcement, when the load is uniformly dis- 
tributed, may be gradually increased from 
the middle third to the outer edges, if the 
slabs of the bars are kept of a uniform size, 
provided that the spacing at the outer edges 
be not greater than three times the spacing 
at the centre of the slab. 

Calculating Slab Supported on Four 
Sides. As an example of slab design, let 
it be required to calculate the thickness and 
reinforcement for a slab 12 ft. by 8 ft., sup- 
ported on all four sides to carry a load of 
275 Ib. per square foot in addition to its 
own weight. The stresses in the concrete and 
steel not to exceed 600 Ib. and 16,000 Ib. 
respectively. 

The weight of the slab itself will require 
to be found in the first instance, as this 
must be added to the external load in order 
to ascertain the bending moment. Assume 
a total thickness of 6 in. for this purpose, 
and the weight per square foot will then be 
1 ft. x 1 ft. x -5 ft. x 150 Ib. = 75 Ib. 
The total load per square foot is equal to 
275 Ib. + 75 Ib. = 350 Ib. It is now re- 
quired to calculate a portion of the slab, 
8 ft. long and 1 ft. wide, and the area will 
equal 8 sq. ft. The total load on this por- 
tion will be the area multiplied by the weight 
per square foot ; thus 8 x 350 = 2,800 Ib. 
total load. The bending moment can now 
be calculated, bearing in mind that the slab is 
supported on all four sides, and the formula 



i /A/\4 must be employed. There- 



THEORY OF REINFORCED CONCRETE 



97 



fore the bending moment 
2800 x 96 in. 1 



will equal 



1 + 



BM = 33600 x 



1-197 



B M = 28,070 Ib. 

Having now obtained the bending moment, 
the depth can be calculated from it by the 
formula previously given, namely : 

/BM 
= V i 



d 



1140 



d = V 2 . 4 ' 8 
d = 4-9 in. 

To use the economical percentage of rein- 
forcement, the sectional area must be made 
equal to -00675 6 d, and this, therefore, will 
be -00675 x 12 x 4-9 = -3969 sq. in. The 
area of one -in. round rod equals -1963 
sq. in., and two rods will give a sectional 
area of -3926 sq. in. This is such a small 
fraction under the required amount that 
it can be used, and as d was found to be 
equal to 4-9 in., this may be increased to 5 in. 
for practical purposes. As there will be 
two ^-in. rods for every foot in width, they 
will be spaced at 6-in. centres, and this will 
be quite satisfactory. By adding 1 in. of 
concrete to the effective depth of 5 in., in 
order to afford covering for the reinforce- 
ment, the total depth will be equal to 6 in., 
which exactly agrees with the assumption 
made in the first instance. 

It will now be necessary to check the 
stresses in the steel and concrete to see that 
they do not exceed the permissible limit. 

The weight of the slab having been cor- 
rectly assumed, it will not be necessary to 
recalculate the bending moment, as in the 
case of the beam designed in the previous 
example, and the position of the neutral 
axis is the only unknown factor. This will be 
found by using the formula already given : 

/2 A t md /At m\ 2 A< m 
V ~b + \~b ~) ~~b~ 
/2 x -3926 x 15 x 5 /-3926 x 15\ 2 

= v IT- - + (~^-) 

3926 x 15 

= y x 4-9075 + (-49075) 2 - -49075 
= y/ 4-9075 + : 2408 - -49075 
- v/ 54483 - -49075 
- 2-26 - -49075 
= 1-769 

7 



It is interesting to compare this value for 
n with that which would be given by taking 
same as -36 d, such as would be the case if 
the stresses were known to be 600 and 
16,000 Ib. respectively ; then it would equal 
36 x 5 in. = 1-8, which is very near. 

To check the stress in the concrete will 
be the next step. 

2BM 



c = 



bn 



x I a 7: 



c = 



2 x 28070 



12 x 1-769 x 
56140 



C = , r~ 



_ 56140 
* C 93-4 



12x1-769x4-4 
c = 601 Ib. 

This can be considered as satisfactory, and 
the stress on the steel be checked by the 
formula : 

BM 

t = 



t = 



At x Id 

28070 



28070 
1-727 



3926 x 4-4 
t = 16,253 Ib. 

This is slightly in excess of the permissible 
limit, and to overcome this the spacing of 
the bars may be slightly reduced, say from 
6-in. to 5-in. centres. The student is advised 
to do this, and again check the value of t, 
as these examples are merely given for the 
purposes of illustration, and constant prac- 
tice and experience will be the best asset of the 
designer when he has once mastered the prin- 
ciples and formulae which govern his design. 

It is necessary now to find the longitudinal 
reinforcement that will be necessary, and for 
this purpose the bending moment must be 
worked out for the longer span. As shown 
above, the total load per square foot = 
350 Ib., and the load on a strip 1 ft. wide 
and 12 ft. long will therefore equal 350 x 12 
= 4,200 Ib. The bending moment will be 
found by the formula for the longer span , 

i.- -u wl l 

which is -tr x 



BM = 



4,200 x 144 in. 



1 +1-= 



BM = 75COO x 
BM 



1 + 5-06 



75600 



6-06 



= 12,475 in.-lb. 



9 8 



REINFORCED CONCRETE 



It will not be necessary in this instance to 
calculate the depth, as this is given by the 
calculations for the shorter span, when it 
was found to be 5 in., and as the longitudinal 
reinforcement will be placed on the top of 
the cross-rods, d will be reduced by the dis- 
tance between the centres of the two sets 
of rods, which will be in. if the longitudinal 
rods are of the same diameter. It will be 
better to keep them the same size if pos- 
sible and space them farther apart. The 
depth will then be 5 in. -5 in., which 
equals 4-5 in. In order to make quite sure 
that this is sufficient, d may be calculated 
according to the bending moment given and 
the results compared. As before : 

: 



1UO 



12470 
1140 



d = 



d = VlO = 3-1 in. 

This is less than the value already obtained, 
and the amount of 4-5 in. may be worked to, 
which will have the effect of reducing the 
amount of reinforcement required, as the 
lever arm of the internal resistances will be 
increased. This being so, seek for some 
method of deducing the value of At when 
d is given and is in excess of that required 
by calculating same from the bending 
moment. The bending moment is known, 
the depth is known, and n should be equal 
to -36 d if the stresses are to be satisfactory, 
and the formula for finding t, which is 

BM 

t = - / r, can be used. If, how- 



At x (d - 

ever, a value is given to t of 16,000 Ib., then 
there is only one unknown factor, namely 
A t , and this can be found as follows : 



16000 = 



A, = 



-36 d 



12475 



16000 x (d - -12 d) 

12475 
16000 x -88 d 

12475 



12475 
63360 



16000 x -88 x 4-5 
A t = -1968 sq. in. 

The area of one -in. diameter bar equals 
1963 sq. in., and therefore this size can be 
employed and spaced at 12-in. centres, 
which is the maximum permissible limit for 
tensional bars. 



Checking the Slab. It will now be 
necessary to check the stresses in the con- 
crete and steel, and in order to do this the 
position of the neutral axis must first be 
found as follows : 



2 A t m d 



/A^mY- 
I b~) 



A t m 



/2x -1963x15x4-5 /-1963xl5 
: V ~i2~~ ' \ 



12 



1963 x 15 



12 



n = v/2-2083 + (-245S) 2 - -2453 
n = x/2-2684 - -2453 
n = 1-5 -2453 = n = 1-254 in. 
For checking the stress in the concrete : 
2BM 

C = - T- 

bn x (d 



c = 



2 x 12475 



12 x 1-254 x 
24950 



c = 



24950 
61425 



12 x 1-254 x 4-082 
c = 406 Ib. 

This is well below the permissible limit, 
but t should be checked, and this will pro- 
bably be very near the amount of 16,000 Ib. 

BM 

t = 



-(-9 



12475 



t = 



1963 x 
12475 



1963 x 4-082 
_ 12475 
: 



t = 15,568 Ib. 

This amount is not very much below the 
permissible value, and it will not be advis- 
able, therefore, to decrease the reinforce- 
ment in any way, and the slab can be con- 
sidered as satisfactorily designed. It will 
be impossible to stress the concrete to its 
full extent without overstressing the steel, 
as the depth, which influences the amount, 
of concrete is in excess of that theoretically 
required, as previously explained. Figs. 121 
and 122 show how the dimensions are taken. 
It is interesting to note what the result 
would be if the area of the reinforcement 
had been based on the theoretical depth 
instead of taking advantage of the depth 
given, and for the guidance of the student 



THEORY OF REINFORCED CONCRETE 



99 



this will be worked out. It lias been seen 
that this depth was 3-1 in., and the theoretical 
area of the concrete in section therefore 
equalled 3-1 x 12 in. - 37-2 sq. in. If A t 
was made equal to -00675 b d, then it would 
be 37-2 x -00675 = -2511 sq. in. This is 



6'l d=S' 

/LONGITUDINAL BARS 




Fig. 121. Section Through Gross Rods 

greatly in excess of the amount actually 
required, and as the stresses as shown above 
are even now below the permissible limits, 
it is obvious that they would be far more 
so in this case, and the design would not be 
economical. 

In the case of slabs which are fixed, the 
bending moment at the ends of the spans 
must also be calculated, and reinforcement 
provided on the upper surface to take the 
tension which will occur. This is effected 
by bending up some or all of the bars in 
the lower surface when they are no longer 
required in the latter position ; or short 
bars may be placed on the upper surface 
and carried for the necessary distance into 
the slab. The turning up of the lower bars 
will assist in the resistance to shear, and 
this will be more fully explained and illus- 



i>=/2"- 



2, PODS 

^CgO 

jr- 




12' 



Fig. 122. Section Through Longitudinal Rods 

trated in the notes dealing with this por- 
tion of the subject. It must be borne in 
mind, however, that it will not be sufficient 
to calculate the bending moment in the 
centre of the span only, and although the 
same theory and principles will apply in 
both cases, the student is advised to take 
an example of a fixed slab and calculate 



the reinforcement necessary at the various 
points for the purposes of practice. 

BEAMS WITH DOUBLE REINFORCE- 
MENT 

With regard to the beams and slabs 
described up to the present, no restrictions 
have been placed on the size of the members, 
and consequently the depth in some in- 
stances has been such as would cause great 
inconvenience in buildings of limited height. 
To allow of beams of less depth being em- 
ployed, it is necessary to introduce double 
reinforcement, which consists of two sets of 
rods, one of these taking the tension as 
before, and the other taking a certain pro- 
portion of the compression which the reduced 
quantity of concret3 is unable to resist. 
Another consideration is the weight of the 
beam. In the case of long spans, a beam 
with single reinforcement may be so heavy 
that its weight is greater than the external 
load to be carried ; the introduction of com- 
pressional reinforcement reduces the size of 
the beam necessary and thus reduces the 
total load to be carried. This is an impor- 
tant consideration, as not only is the bending 
moment on the beam reduced, but so also 
is the load to be carried by the supports 
and the foundations. 

The procedure in the design and calcu- 
lations is somewhat different from that 
employed in the case of beams with single 
reinforcement, as with the latter it is neces- 
sary to begin with an assumption regarding 
the size of the beam required, in order to 
calculate the bending moment ; but now a 
definite size can be fixed for the beam at the 
outset and suitable reinforcement inserted. 
This size should be made as near the econo- 
mical section as the circumstances will per- 
mit, if there is any choice in the matter. 
As the size will thus be available at the out- 
set, the weight of the beam can be found and 
the bending moment calculated. This bend- 
ing moment will be the one to be resisted, 
and there will be no variation or re-calcula- 
tion on this point. 

The next step will be to consider the 
strength of the beam according to the fixed 
size by adding the economic proportion of 
einforcement, and considering it as a single 
reinforced beam, and then calculating the 
bending moment that such a beam will 
resist. It will necessarily be less than the 
actual bending moment to be resisted, and 
the difference between the two will give the 



100 



REINFORCED CONCRETE 



excess that has to be taken by the com- 
pressional reinforcement and the extra ten- 
sional bars that will be required over and 
above the economical percentage. 

Figs. 123 and 124 will be some guide as 
to the disposition of the reinforcement, and 
the meaning of the symbols employed. 

It is necessary to introduce a certain 
number of new formulae, owing to the new 
factor in the compressional resistance, and 
these can be explained in the simplest 
manner by taking an example and working 
it out step by step with the necessary 
explanation as it becomes due. 

Designing Beam with Double Rein- 
forcement. A beam is required to carry 
a uniformly distributed load of 10 tons in 



covering the reinforcement, and this can be 
taken at 2 in., thus giving an effective depth 
of 20 in. and a breadth of 12 in. 

The economical amount of reinforcement 
will be found by multiplying the effective 
sectional area of the concrete by -00675, 
thus 20 x 12 x -00675 = 1-62 sq. in. The 
moment of resistance of this beam will be 
equal to the sectional area of the steel multi- 
plied by the safe stress per square inch 
multiplied by the lever arm of the internal 

/ n 
resistance expressed as A t x t x (d 

and the bending moment will equal the 
moment of resistance, therefore 



BM = A t 



d 



x 



- I 

n 



-A 



d-ff 




Fig. 123 Fig. 124 

Figs. 123 and 124. Double Reinforcement in Beams 



addition to its own weight over a span of 
16 ft., while the over-all size is limited to 
22 in. by 12 in. The actual weight of the 
beam must first be calculated in order to 
find the total load. This will equal 1|- x 1 
x 150 lb. x 16 ft. = 4,400 lb., and the total 
load = 10 x 2240 + 4400 = 26,800 lb. The 

w I 
bending moment will equal -?r = B M = 

26800 x 16 x 12 

g - = B M = 643,200 in.-lb. 

The actual bending moment to be resisted 
has thus been obtained, and it is now re- 
quired to ascertain the moment of resist- 
ance of the specified beam if same is rein- 
forced in the tensional area only, and see 
how far it is deficient. As the over-all depth 
is 22 in., an allowance must be made for 



To find the value of n, take this as -36 d 
from previous deductions, therefore n = 
36 x 20 = 7-2 in., and 

BM = 1-62 x leOOO x 

B M = 1-62 x 16000 x 17-6 

B M = = 456192 in.-lb. 
The actual bending moment, however, 
equals 643,200, and the excess which has to 
be provided for by extra tensional reinforce- 
ment, and compressional reinforcement = 
643200 - 456192 = 187008. 

It may assist the student if the moment 
of resistance is worked out with regard to 
the concrete, in order to show that the same 
result is obtained and that the compressional 
reinforcement is necessary. Considering the 
concrete as taking 600 lb. per sq. in., this 



THEORY OF REINFORCED CONCRETE 



101' 



C X 



x n 



= 25920 x 



gives MR = 

600 x 12 x 7-2 

~^T " \~" 3 

17-6 = 456192 in.-lb. as before. It is 
therefore obvious that the compressional 
reinforcement will be required to take the 
excess of 187008 in.-lb. 

The position of this compressional rein- 
forcement must now be settled in order to 
determine the lever arm between the two 
sets of reinforcement. It is advisable to 
place this as far as possible from the neutral 
axis while obtaining a covering of concrete, 
and this will be given if it is situated at a 
distance of 2 in. from the upper surface, as 



= as A/ = 



N 

20' 



12' 



r* 






12' 8 



Fig. 125. Designing Beam Having Double 
Reinforcement 



shown in Fig. 125. There are thus two sets 
of reinforcement, one situated at a distance 
of 5-2 in. from the neutral axis and the 
other at a distance of 12-8 in. from the same 
point. 

The distance from the upper surface to 
the centre of the compressional reinforce- 
ment is expressed by the symbol d c , and 
the distance apart or lever arm of the two 
sets of bars will be d d c . Considering, 
first, the additional tensile reinforcement 
required, the limiting stress, which is 
16,000 Ib. per sq. in., is known, and also 
the lever arm, which is d d c . If, there- 
fore, the excess bending moment is divided 
by t, x (d d c ), the amount of steel neces- 
sary will be found. This may be expressed 



Excess B M 
t x (d - d~Y 



Working this out 



gives 

A/ = 

A/ _ 


187003 




16000 x(20 
187003 


-2) 

/ 


t 


288000 ~ A 


' 



This additional reinforcement must be 
added to the amount already calculated for 
in the tensional area amounting to 1-62 sq. in. 
making a total of 2-269 sq. in. 

The value of the compressional resistance 
must also equal the excess bending moment, 
and if the area of the steel in compression 
be expressed by the symbol A c and the 
intensity of the stress by the symbol c,, 
then A c x c s x (d d c ) = excess B M and 

Excess B M 

c ' '' c (d d Y value of c.,. will 

require to be found before proceeding further, 
as although the stress intensity could safely 
be 16,000 Ib. per sq. in. in compression as 
in tension, it will not be possible to develop 
this amount on account of the steel being 
so much nearer the neutral axis in the com- 
pression area than in the tension area. The 
stress varies directly as the distance from 
the neutral axis, and therefore, if the com- 
pression steel is placed twice as near to the 
axis as the tension steel, then it can only 
develop one-half the stress taken by the 
latter. In other words, the stress on the 
compression steel multiplied by the distance 
of the tension steel from the axis must equal 
the stress in the tension steel multiplied by 
the distance of the compression steel from 
the axis. Expressed by symbols, this reads 
Cg x (d n) =tx (n d c ), therefore o = 

t x (n - d c ) 
-j-^ . In this example 

16000 x (7-2 - 2) 



c* = 



20 - 7-2 



832000 



12-8 

c, = 6,500 Ib. 

Again, this may be shown in another 
way, as when steel and concrete are acting 
together in compression the stress in the 
steel can only reach m times that in the 
surrounding concrete, as it is only under 
these conditions that they can act together. 
The stress in the concrete at the extreme 
upper edge is equal to 600 Ib., and conse- 
quently at a distance of d c from the point 
it will be less, as it is closer to the neutral 



102 



REINFORCED CONCRETE 



cxn- d 



axis. The stress will be equal to 

which expresses proportion of stress at this 
point. The value of c s as above will equal 
m times this amount, and therefore 
m x c x (n d t ) 

15 x 600 x (7-2 - 2) 

7-2 
c s = 6,500 lb., as before. 

Excess B M 

The formula A r = - TJ j-\ can now 

c s x (d - d c ) 

be worked out and the compressional steel 
found, thus : 

187008 
A ' == 6500 x (20 - 2) 

187008 

A ' ~'~~ llTOOO 
A c = 1-598 sq. in. 

The beam as designed, therefore, has an 
over-all size of 22 in. by 12 in., with an 
effective depth of 20 in., and the reinforce- 
ment consists of 2-269 sq. in. in the tensional 
area and 1-598 sq. in. in the compressional 
area. 

BEAMS AND SLABS WITH DOUBLE 
REINFORCEMENT 

In checking the stresses in a beam already 
designed and containing double reinforce- 
ment, it is natural that the formulae should 
be a little more complicated than in the 
case of single reinforced beams, as there are 
more factors to consider in the resistance. 
In the case of the stress on the concrete 
there is some portion of the compression 
taken by the steel in the upper part of the 
beam, and this, obviously, must be con- 
sidered in arriving at the value of c. It will 
be necessary to see how the formula for 
finding c is constructed. As a basis for 
the reasoning, there is the theory already 
evolved, namely : 
cbn I n\ 
qr~ x ( d Q I and A c x c s x (d - - d.) 

Zi \ O / 

together equal B M. This can be stated as 
follows : 

2 B M = c b n (d - * ) + 2 A c . x c, x (d- d f ) 

\ 6 ' 

The factor c s will be unknown until the 
compression stress on the steel is found, but 
its equivalent value, as previously shown, 

c x m x n d c 

is - , and this can be sub- 

n 

stituted, giving : 



2 B M = c 6 w x 



M 



+ 2 A c c m x 



Therefore : 
2EM. 



= clbn x (d - ^ + 2 A c 

'-'] 



m x 



and c will be found by the formula : 

c = 

_ 2BM _ 

b n x ( d - 1 j + 2 A c m x ( - C J x (d- d c ) 

This formula appears to be a very large one, 
but it is perfectly simple, and will very 
quickly become familiar to the student, 
who, however, will be unable to make use 
of the equation until the value of n is known ; 
and it will be necessary to deduce a formula 
for this, based on the relative values of the 
concrete, steel in compression, and steel in 
tension. 

It is known that the total compression and 

cbn 
tension are equal, and therefore -~ - + A c 

x c s = t A t , or c b n + 2 A c x cs = 
2tA t . 

We do not know the values of t, c, or c s , 
but can replace them with equivalents as be- 

t m x d n 

, as shown by 
n 

c m (n d c ) } 



fore ; thus - = 



previous reasoning, and c s = 



c s m 

OI J _ 

c n 



d) 
ci 



x c s = 2 t A t , and if we divide both 
sides of the equation by c we shall get 

2 A c x c, 2tA t 
bn + - ~ , or 6 M + 2 AS x 



c s . 



t 



t C- 

By substituting the values of - and - as 

C C 

given above, we can remove the unknown 
factors and obtain the following : 

2 A c m (n d c ) 2A t m(dn) 

bn + ~Y~ ~^~ 

Multiply both sides of the equation by n and 
we have 

b n 2 + 2 A c m (n d c ) = 2 AC m (d n). 
We have now got n on both sides of the 
equation, and this must be simplified as 
follows : 



THEORY OF REINFORCED CONCRETE 



103 



b w 2 + 2 AC m n 2 A c m d c 2 A t md 

2 A t mn 
bn 2 + 2 A c mn + 2 A t mn = 2 A t md 

+ 2 AC m d c 

bn z +2mn(A c + A t ) = 2m(A t d+A c d c ) 
2 mn (A c + A t ) 2 m (A t d + A c d c ) 



,, 

If we now add 



, T, -A 

to both sides 



of the equation it will be possible to take the 
square root of the left-hand side and further 
simplify as follows : 

2mn(A c + A t ) fm (A c H 
~~b~~ ~ L b 

2 m (A t d + A c d c ) , f m (A c + I 



J 






The square root of the left-hand side equals 
n + -%- , therefore this will equal 



/2m(A t d+A c d c ) , 


m (A c + A t ) 


2 

, and 


V 6 


b J 



2 m (A t d 4- A c d c ) 

b 

m (A c + A t 



f 



m 



Having now obtained a formula for the 
position of the neutral axis, it will be possible 
to ascertain this and then calculate the 
stress on the concrete. When this has been 
done the stress on the steel can be found by 
considering the elasticity of the two mate- 
rials and their relative distances from the 
neutral axis. Thus t will equal m times c 

multiplied by - , or, expressed as a for- 



d 



and c s will equal 



mula, t = c m 

n d,, 
cm . 
n 

It will be advisable to give an example 
illustrating the use of these various for- 
mulae for the purpose of checking ; the case 
taken will be that of the beam previously 
designed to carry a load of 10 tons distri- 
buted in addition to its own weight over 
a span of 16 ft., when the effective depth 
was limited to 20 in. and the width to 12 in. 
The tensile reinforcement necessary was 
calculated to be 2-269 in., and the com- 
pressional reinforcement 1-598 in. 

There will be no need to re-calculate the 
bending moment, as the size, and conse- 
quently the weight, of the beam was avail- 
able at the outset. This bending moment 
was found to amount to 643,200 in.-lb. The 



first step will be that of finding the position 
of the neutral axis. 

2 m (At d + A c d e ) fro (A c +A)1 2 



m(A, ; +A t ) 



[~m(A c +A)"p 
b 



n = 



X 15 (2'269_ XJ20j+_r6ij8j><_2)_ , [15 (1'598 



12 

15 (1-598 + 2-269) 
12 



, 



30 x 48-576 



+ (4-833) 2 - 4-833 



n = N/121-44 + 23-357 - 4-833 

n = x/144W - 4-833 

n = 12-03 - 4-833 

n = 7-197 in. 

Upon reference to the figures taken when 

designing the beam, it will be seen that n = 

7-2 in., and this will afford some evidence 

to the student that the formula is satis- 

factory. The stress in the concrete can now 

be ascertained as follows : 






2BM 



b n x (d - o) + 2 Ac m x ^ ^ x (d- d c ) 

c = 

2 x 643200 

(7'197\ /7'197-2\ 

20--g-)+2x 1-598 x l *x(rriW ) x (2 ~ 2 

1286400 

c = 12 x 7-197~xl7-6 +2 x 1-598 x 15 x -71 x 18 
1286400 



c = 



1520-006 + 612-673 
1286400 



2132-679 
c = 603 Ib. 

This is quite satisfactory, as it is only 3 Ib. 
over the permissible limit of 600 Ib. per 
square inch, and even this would not occur 
if all the decimal figures were retained in 
the calculations, but such a method would 
entail a great deal of unnecessary labour. 
The stress in the tensional reinforcement 

d n , 

= c m - ; therefore 

20 - 7-197 
t = 603 x 15 x -- 7497" 

t = 603 x 15 x 1-77 

t = 16,000 Ib. 

This can be considered satisfactory, and the 
stress in the compressional reinforcement 
will now be calculated by : 



n - 



c s = cm 



104 



REINFORCED CONCRETE 



c s = 603 x 15 x 



7-197 - 2 



7-197 

c a = 603 x 15 x -72 

c, = 6,512 Ib. 

The beam then, as designed, can be con- 
sidered as satisfactory. It will be noticed 
that the exact areas of the steel as calcu- 
lated were retained and not substituted by 
actual practical size bars, as these would of 
necessity vary slightly, and this would affect 
the stresses. The object of this example has 
been to avoid all complications, as the 
theory is a little more difficult than with 
single reinforced beams, and the student 
should become quite clear on all the points 
in the formulae and then work out various 
examples and check them, in some cases 
working out the bars required, and substi- 
tuting these for the theoretical areas before 
finding the stresses in the concrete and steel. 
All the theory and formulae given will 
apply equally to slabs and beams, bearing 
in mind that the bending moments in the 
case of the former must, if necessary, be 
calculated in accordance with the rules pre- 
viously given if supported or fixed on all 
four edges. 

TEE BEAMS 

These beams are those in which the com- 
pressional resistance is partly or wholly 
supplied by the slab above the beam, form- 
ing a large T-shaped section. Such a method 
of calculation is permissible and economical 
when the slab and beam are cast in one 
operation with no plane of cleavage between 
the two, and reinforcement is provided to 
resist the shearing stress occurring along 
the plane of junction. 

When designing a reinforced concrete 
floor over a very large area it is necessary 
to divide the floor up into a number of 
panels by means of beams which will of 
necessity project down below the slabs 
which form the panels, and sucli beams will 
generally consist of main and secondary 
beams, the former spanning the greater 
distances and carrying the latter, which 
intersect at right angles. 

In smaller floors the design may consist of 
one set of beams only, and in this case the 
slabs are considered to assist in the resist- 
ance of such beams, while in the case where 
two sets are employed it is usual to con- 
sider the secondary beams only as being 
assisted by the slabs, and it is essential that 
the slab reinforcement, which is at right 



angles to the beam, must extend through the 
full width of that portion of the slab which 
forms the compressional flange of the beam. 
The width of slab that can be taken as 
acting with the beam will depend on cir- 
cumstances, but it must not exceed either 
of the following : (1) One-third of the effec- 
tive span of the tee beam, (2) three-fourths 
of the distance from centre to centre of the 
ribs of the tee beams, (3) fifteen times the 
thickness of the slab, (4) six times the width 





' 1 


I 

V 


i 




. ' ' 


__^4 


} 

-A- 


N 5 


c 


r 

L 


-- 


1 
' 


A 




B 



Fig. 126. Section of Tee Beam and Method 
of Finding Total Compression 

of the rib of the tee beam. It will be neces- 
sary, therefore, to determine which is the 
least of these and calculate accordingly. 

The diagram presented by A, Fig. 126, 
shows the section of a tee beam, and will be 
helpful in explaining the value and meaning 
of the various symbols employed. 

There are three cases that require to be 
considered according to the position of the 
neutral axis, and these are indicated on 
this diagram. The first is that when the 
neutral axis falls within the slab, the second 
where it coincides with the bottom of the slab, 
and the third where it falls below the slab. 



THEORY OF REINFORCED CONCRETE 



105 



The two former cases offer no difficulty, 
as all the formulae that have previously been 
given for beams with single reinforcement 
can be employed, with the exception that b 
will now represent the width of that portion 
of the slab acting as the flange of the beam, 
and in the second case d s , which expresses 
the thickness of the slab, will be equal to n. 

In the third case, however, it will be seen 
that a portion of the rib will be acting in 
compression, and this will affect the centre 
of the compressional resistance, and conse- 
quently the lever arm of the internal forces. 

In most cases the amount of concrete in 
the rib above the neutral axis which assists 
in compression is neglected in the actual 
calculations, as the area over which the 
stress is acting is comparatively small, and 
the resistance of such a small area situated 
so closely to the neutral axis is not such as 
to materially affect the economy of the 
design. The second report of the R.I.B.A. 
recommends that this method of calcula- 
tion should be adopted, and it is therefore 
justifiable to consider the compression 
strength in this way. 

Upon reference to B, Fig. 126, it will be 
seen that the total compression is represented 
by the shaded area, which has a depth equal 
to d s and width equal to c at the extreme 
top edge and c 1 at the bottom edge, which 
coincides with the bottom of the slab. The 
area of the beam over which the stress is 
considered as acting is equal to b x d s , 
where 6 equals the width of the flange and 
d 3 the thickness. Now. the total compres- 
sion in the slab will be the area 6 x d s mul- 
tiplied by the stress per square inch. As 
before explained, the stress diminishes uni- 
formly towards the neutral axis, and conse- 
quently the value of c 1 will be less than c in a 
direct ratio with its comparative distance 

from n. Therefore c 1 = c x s , and 

n 

the mean stress acting over the whole area 

n d s 
c x c 1 c + c 



will equal 



or 



If the 



area acting in compression be multiplied by 
the mean stress per square inch, then the 
total compression will be given. As before 
stated, the area =b x d s , therefore the total 
compression = 

, / n - d s \ 
o x d s x (c + c 1 



The moment of the compression must equate 
with the bending moment, and the total 
compression will therefore be required to be 
multiplied by the lever arm of the internal 
forces. In the case of singly reinforced 

1% 

beams this was always equal to d , but 

this will no longer apply, as the centre of 
the compressional resistance will no longer be 

situated at a distance of ^ from the upper 

surface. The symbol a, however, is used to 
express the value of the lever arm, and this 
can be employed to complete the formula 
for the moment of the compressional resist- 
ance, which will be equal to 

/ n d s \ 

ox a s x Ic + c - 

\ f 11' 

s x a, and this 

must equal B M. 

From this formula may be deduced a 
formula for finding the value of c when the 
bending moment has been calculated, as 
follows : 



/ i 
c 6 x d s x 1 2 


4^') 


x a 

"R M 


2 
c [6 x d s x (2 n 


- 4) x 


a l BM 



2BMn 



Therefore c = g x rfg x (2 n _ d .) xa 

The value of the total compression has 
now been obtained, neglecting the concrete 
in the rib, and also the method of finding 
the value of c when the beam is designed. 
It is now necessary to show how the lever 
arm of the internal forces can be ascertainel, 

n 
as it will not be d .7, as m previous cases, 

owing to the total compression, as shown at 
B, Fig. 126, no longer being represented by a 
triangle. As before, the whole of the com- 
pression must be considered as acting at 
the centre of gravity, and it is necessary, 
therefore, to find the centre of gravity of the 
shaded portion, shown on the diagram. 
The draft regulations of the London County 
Council state that the lever arm can be 

taken approximately as d ^. This is a 

useful rule for preliminary calculations when 
the steel is required to be found and the 
position of the neutral axis is not avail- 
able, but an accurate method must be 
deduced for exact calculations. Now if 



io6 



REINFORCED CONCRETE 



the shaded portion was a triangle having a 
base equal to c and a height equal to d s , 
then the centre of gravity would be situated 

d s 
at a distance of -^ from the top edge. It is 

necessary to consider, however, a figure 
having four sides, where c 1 will bear a cer- 
tain relation to c, and it is obvious that the 

d, 

centre will be nearer to the top than w> 

and by the principle of moments it will be 
found that the actual position of the point 

d s 2 + 2 c 1 
from the top edge will equal ^ x ^. 

Express the distance from the top edge by 
the symbol a , as shown in Fig. 126. It has 
previously been seen, however, that c 1 = 

n dg 

c , and by substitution the following 



is obtained : 



c + 2 c x 



n d s 



a, = 



, 

3 c n - - 
n 



2cn 



c + c x 



n 



= x 



f 
Therefore a e = 



3 n-2d s _ 3 d s n - 2 d, 2 
2 w d 6 n 3d s 



j . i 

> and the lever 
' 



6 n 
arm of the internal forces will equal d a c 

3d s n -2d s * 
- 



The total tension will equal t x At, and 
the, tensional resistance will be this value 
multiplied by the lever arm, namely t x At 
x a, where a = d a c . This tensional resist- 
ance must also equal the bending moment, 
and therefore t A x d a c = B M, and the 
stress in the steel can be found in a beam 
that has already been designed by the formula 



A t x d a c ' 

Having ascertained the methods of deal- 
ing with both the compressional and ten- 
sional resistances, the next step will be to 
deduce some process for finding the position 
of n in the case of a beam already designed, 
and this is of great importance, as there are 
three possible cases, as stated previously. 
In order to be quite accurate, it will be 
necessary to consider the concrete in the rib, 
as it is obvious that any compression in this 
portion would affect the position of n, and 



although it is neglected in the actual cal- 
culations for strength, it must be taken into 
account in the neutral axis formula. The 
compression in the slab has been shown to 
be equal to 



and to this must be added the compression 
on the leg. This will amount to 

n d s 
b >< C X 



where b r is equal to the width of the rib, 
giving a total compression of 

n d.\ in d. 



and as the total compression and tension are 
equal, thus 

n d A In d s 



This can be simplified and deduced to give 

a value for n as follows : 

2cb d s n cb d? + c b, n cb r d s 

2~ ^^ t A.f 
n 

c (2 b d s n b d s z + b r n b, d s ) 

2n 

2 t At n 
2bd s n 6 d s 2 + b n b d s = - 



t m(d 

but - = 

c n 

get: 

2 b d s n bd s z 



n) 



, and by substitution we 



z + b r n b r d s = 
2m A t n (-I n) 



n 



2 b d s n b d s 

2m A t nd 2m 



bn b r d s = 



2bd s n bd s * + b r n b r d s = 2 m A t d 

2mA t n 
2 b d s n + b r n + 2 m A t n = b d s 2 + b r d, 

+ 2 m A t d 
n [2 (b d s + m A,) + b r ] = d s (b d s + b f ) 

+ 2 m A t d 

d, (b d, + b r ) + 2 m A, d 

therefore n = o~71Tj i -- /T\ i iT~ 
2 (b d s + m A t ) + b r 

Thus all the formulae necessary is obtained 
for the checking of beams which are already 
designed, and it is only necessary to ascer- 
tain the method to be followed when design- 
ing the beams to carry a stated load, and 
this will now be given. 



THEORY OF REINFORCED CONCRETE 



107 



In the designing of tee beams, there are 
certain factors which will be available from 
the previous calculations, and it will be 
advisable to note these in the first instance. 
The width of the compression flange, together 
with the thickness, will be given, as the slabs 
will have been designed previously, and, 
consequently, the values for b and d s will 
be known. There remains for consideration 
the method of finding the thickness of the 
rib ; the value of d required to give econo- 
mical design ; and the amount of reinforce- 
ment. The first-mentioned, namely, the 
width of the rib, will generally be fixed by 
the designer according to experience, as it 
will be influenced by the fact that the rods 
in the tensional area must be properly spaced 
and covered ; and as the value of the 
compression in the rib is neglected even 
when the neutral axis falls below the slab 
the calculations for the design will not 
be affected. There is a minimum limit, how- 
ever, and that is, the width must not be 
less than one-sixth of the width of the com- 
pression flange, but it is seldom that this 
minimum will be employed. 

The method of finding the economical 
depth is one that calls for a little more con- 
sideration. In the first instance, it is not 
known whether the neutral axis will fall 
below the slab or not, and if it should be 
found to do so after the design is complete, 

% 

then the lever arm will not be d > ^ ut 

d a c , and, unfortunately, no definite rela- 
tion between a c and d can be fixed unless 
the amount of reinforcement, and conse- 
quently the position of the neutral axis, is 
known. To keep matters as simple as pos- 
sible, however, it will be advisable still to 
consider n as equal to -36 d, as will be the 
case if it falls within the slab, and to con- 
struct a formula for finding d accordingly ; 
and if the value for d thus found is not 
quite accurate, it will form the best guide, 
and the necessary adjustment can be made 
after. 

In previous notes it has been shown that 

cbn / , n\ . 

BM = o- x Id o ) for a single rein- 
forced beam, and if the neutral axis is 
assumed to fall within the depth of the slab, 
then this formula will hold good. In de- 
ducing a formula for giving the value of d, 
values may be substituted for c and n, but 
6 will be unknown, as it will vary in practic- 



ally every example, and will not necessarily 
bear any definite relation to d. By putting 
in the values already known, it can be 
deduced as follows : 

( ~\ 

-ex x n * \ - 3) 

c = 600 and n = -36 d, then 



2 B M = 600 x b x -36 d x I d - 

2 B M = 600 x b x -36 d x -88 d 
2 B M = 180-08 b d 2 
B M = 90-04 b d 2 

,72 _ BM , _ /"BIT 

= 90-04 b ~ : V 90-04 b 
It will be sufficiently accurate if the deci- 
mal figures are deleted, giving 



906 

With regard to the method of calculating 
the amount of reinforcement required, or 
the value of A/, this again presents difficul- 
ties, as it will depend upon certain factors, 
one of which will be unavailable, namely, the 
lever arm ; and although it may be taken as 
being equal to -00675 6 d when the neutral 
axis falls within the slab, this method will 
not be absolutely accurate when the axis 
falls below the slab. Another method will 
be that of assuming the value of the lever 
arm, and for the preliminary calculations 

d s 
this may be taken as equal to d <r when 

a safe approximation will result under 
ordinary circumstances. If this method is 
considered and accepted, then A can be 
found as follows : 

B M = A t x t x a, where a = the lever 

d st 
arm, and this is assumed as equal to d -5 

/ d s \ 

therefore B M = A x t x I d % I, 

BM 

then AC = - 



TEE BEAMS CONTINUOUS OR FIXED AT 
ENDS 

In the case of tee beams that are continu- 
ous or fixed at the ends, it will be necessary 
to calculate the strength at the supports ; 
and as the flange at these points will be in 
the tensional area, its value will be lost and 
the calculations must be made for a rect- 
angular beam having a width equal to the 
width of the rib only. If an example is now 



io8 



REINFORCED CONCRETE 



taken, and the beam is designed and checked 
according to the theory and formulae given, 
the student will be able to realise the applica- 
tion quite clearly. 

It is required to design a tee beam, fixed 
at the ends, to carry a uniformly distributed 
load of 2,000 Ib. per ft, run, including the 
weight of the beam itself, over an effective 
span of 20 ft.- The slab has been designed, 
and has a total depth of 6 in., while the 
width acting in compression with the beam 
can be taken at ten times d s , which equals 
60 in. 

The total load will equal the load per foot 
run multiplied by the effective span, thus 
w = 2000 x 20 = 40000 Ib. As the beam 
is fixed at the ends, the bending moment at 
the centre can be considered as equal to 

wl 40,000 x 20 x 12 

-jg, then B M = jg 

800000 in.-lb. 

The first step in the actual design of the 
beam will be to calculate the depth by the 



formula i. 



B~M 



t 

then i - 



= d = x/148 = say 12 in. 

Let us next find the value of A t by con- 
sidering same as -00675 6 d. Then A t = 
00675 x 60 x 12 = A, = 4-86 in. If the 
value of the steel required is also calculated 
by the alternative approximate method of 

considering the lever arm as equal to d - e 

a 

then the use of the formula will be shown. 
This formula was A/ = 



Therefore A t = 



A, = 



800000 
16000 x (12 - f 
800000 



16000 x 9 
At = 5-55 in. 

The first method, which gave 4-86 in., is 
therefore the most economical, provided the 
permissible stresses are not exceeded, and 
this will be ascertained by checking the 
beam. Should the steel provided by the first 
method be found insufficient, then it may 
be increased by any amount up to that given 
by the second method, when it will always 
be found to be sufficient. 

In checking the beam the position of 
the neutral axis must first be found, 
and the formula for this has been given, 
namelv : 



(b d, + b r ) + 2mA t d 
2(bd, + mA t ) + b r ' 
Assuming b r to be 10 in., then 

6 (CO x 6 + 10) + 2 x 15 x 4-86 x 12 



_ 



2 (60 x 6 + 15 x 4-86) + 10 
370 + 30 x 58-32 



_ 



2 x (360 + 72-9) + 10 

2220 + 1749-6 3969-6 

=n=-. l .. s-s 



865-8 + 10 
The thickness of the slab was given as 
6 in., and the distance from the top edge 
to the neutral axis is 4-58 in., therefore the 
latter falls within the slab, and the beam 
can be considered as a singly reinforced beam 

Al 

when the lever arm equals d - Q . 

o 

stresses can be checked as follows : 

2BM 

c = - ^ ^ ; therefore 

b n : 



The 



2 x 800000 



c = 



CO x 4-58 x 

1600000_ 

60 x 4 : 58 x 1048 
1600000 



( - 4 



2879-9 
c = 555 Ib. 

This is quite satisfactory, and t can now 
be calculated. 

BM 

t = - -, -^r ; therefore 



A t x ( d - ^ 
800000 



t 



t = 



4-86 x 112 - 
800000 



4-86 x 10-48 
800000 



50-835 

t 15737 Ib., which is also quite 
satisfactory. 

The steel required was 4-86 in., and the 
area of one bar having a diameter of 1| in. 
= -994 in., and five bars of this size would 
give an area = -994 x 5 = 4-97 in., which 
will do very well. As five rods are to be used, 
it will be necessary to put them in two rows 
one over the other, and the depth of 12 in. 
will be the distance from the top edge to 
the centre between these rows. In order 
to get sufficient covering for the lower rods, 
it will be necessary to add 3 in. of concrete, 
giving a total depth of 15 in. 



THEORY OF REINFORCED CONCRETE 



109 



In this example the beam would require to 
be calculated at the ends, as it was stated 
to be fixed, and this would be done by using 
the formulae given for doubly reinforced 
beams. As this example, however, gave 
an instance where the neutral axis fell within 
the slab, it will possibly be more useful 
to give another example where the axis falls 
~3elow the slab and calculate the section at 
the centre and the ends also, and the student 
will then have an illustration of the procedure 
in each case. It must be borne in mind 
that these examples are taken at random, 
and, as before stated, are given for the sole 
purpose of showing the application of the 
formulae and the methods adopted, and the 
reader will be well advised if he studies the 
working drawings of reinforced concrete 
work, and endeavours to follow the prac- 
tical application of the theory. It is some- 
what difficult to select an example where the 
ixis will fall below the slab unless the load 
to be carried is made exceptionally heavy, 
sr the compression flange is taken as being 
rery thin and narrow ; and if the load is 
sxcessively heavy and the beam is fixed at 
the ends, then a large amount of reinforce- 
ment will be required in the compressional 
area at the ends where the flange is situated 
in the tensional area, as the steel will not 
be situated at any great distance from the 
neutral axis and cannot be stressed to its 
full limit. 

As an example, however, let it be required 
design a tee beam securely fixed at the 
mds, to carry a uniformly distributed load 

2,551 Ib. per ft. run, including the weight 
)f the beam itself over an effective span of 
14 ft. The section at the centre of the span 
is to be calculated in the first instance, and 
then the section at the ends, and the slab 
in be taken as 4 in. thick, and the width, 
icting with the beam, as ten times d s , 

hich equals 40 in. 

The total load will equal the load per ft. 
m multiplied by the span = 2551 Ib. x 
L4 ft. = 35714 Ib. 

The bending moment at the centre will 
wl 35714 x 14 x 12 

jqual j---, therefore B M = - jc> 

= 499,996 in.-lb., say 500,000 in.-lb. 
First calculate the depth, namely : 



BM 



therefore d 



P 

= V of 



500000 



90 b " 'V 90 x 40 

= -s/139 = 11-8 in., say 12 in. 
The value of A t must next be found by 



taking same at -00375 b d ; therefore A t = 
00875 x 40 x 12 = 3-24 in. 

The section at the centre is now obtained, 
and before proceeding with the calculations 
at the ends of the span it will be advisable 
to find the position of the neutral axis, 
and see that this falls below the slab, and 
also check the stresses on the steel and 
concrete. 

The position of n will be found by the 

, d, (b d, + b r ) + 2 m A t d 

formula n ,, j 1-\ rr 

2 (b d s + m A t ) + b,. 

There is only one unknown factor that must 
be settled before working out the formula, 
and that is the value of &,.. Taking this, as 
before, at 10 in., then 

4 (40 x 4 + 10) + 2 x 15 x 3-24 x 12 



n 



n = 



n = 



n = 



2 (40 x 4 + 15 x 3-24) + 10 
4 x 170 + 30 x 38-88 

2 (160 + 48-6) + 10 
680 + 1166-4 

427-2 
1846-4 



427-2 
n = 4-32 in. 

As the thickness of the slab is only 4 in., 
it will be seen that the axis falls -32 in. 
below same, and although this is a very 
small distance, it will serve as an example. 

The position of the neutral axis having 
been obtained, the next thing will be to 
check the stress in the concrete, and as the 
axis falls below the slab it will be necessary 
to use the formula 






6 x d s x (2 n d s ) x a 
The factor a is at present unknown, as this 
represents the lever arm of the internal 
forces, and this must be found in the first 
instance. It is known that a d a c and 

3 d, n - 2 4 2 .. , 
a,. = - S-T , therefore 



6 n 
3 



x 4 x 4-32 - 2 x 4 x 4 



a,, = 



6 x 4-32 - 3 
51-84 - 32 



fit- f\r. 



x 4 
19-84 
W92 



25-92 - 12 ~ 
a c =T42 in. 

As a comparison, it is interesting to note 
that if the neutral axis had coincided with 
the under side of the slab, the centre of 

compression would have been equal to ^ 

which would give a distance of 1-44 in. from 
the top edge ; and as the axis falls below the 
slab by a distance of -32 in., the centre of 



no 



REINFORCED CONCRETE 



compression is -02 in. nearer the top edge, 
and consequently the lever arm is increased 
by this amount. As already stated, a =d a c , 
therefore a = 12 in. 1'42 in. = 10-58 in., 
and this value being now available, c can be 
found as follows : 

_ 2 x 500000 x 4-32 

~ 



c = 



40 x 4 x (2 x 4-32 

4320000 

160 x 4-64 x 10-58 
4320000 



_ 
4) x 10-58 



c = 



7854-592 
c = 549 Ibs. 
This is well below the permissible limit, 



L 


b-J 


n" =1 


r 




1 f 

ds-4' 


t 


1 


1.32 


fcl 


_, * 




'Z'COVIR.F 


^ br 




. A 
C/-/2" 

.1 



Fig. 127. Section, at Centre of Span, of 
Tee Beam Fixed at Ends 



and the value of t can be next calculated by 

BM 

the formula t = -i when a again 

J\t X Ct 

= d a c , which = 10-58 in., therefore 

500000 , _ 500000 _ 

= 3-24 x 10-58 ~ 34-2792' l 
Ib. This also is well below the permissible 
limit, and the section at the centre may be 
considered as quite satisfactory. If two bars 
having a diameter of \\ in. are used, then 
the sectional area of steel will equal 1-7671 
x 2 = 3-53 sq. in., which is more than 
sufficient. 

The section as designed is illustrated in 
Fig. 127, and the calculations for the sec- 
tion at the ends of the span may now be 
considered. In this latter case, a different 
bending moment will exist having a value 

wl , __, 35714x14x12 

equal to -TQ, therefore B M = - j~ 

= 599995 in.-lb., say 600000 in.-lb. This 
is a greater bending moment than that 
required to be resisted in the centre of the 
span, and as the tension will now be in the 
upper surface, a comparatively large amount 
of reinforcement will be required. The 
section will be calculated as a beam with 



double reinforcement where the size of the 
concrete is fixed. The portions of the slab 
on each side of the beam must be neglected, 
leaving a section 14 in. deep and 10 in. wide. 
Allowing 2 in. from the top edge to the 
centre of the reinforcement, the effective 
section becomes 12 in. by 10 in. = 120 sq. in. 
The economical percentage if singly rein- 
forced would be -00375 x 120 = -81 in. The 
bending moment that such a beam would 

/ n\ 
resist will equal A t x t x Id ^j and n 

would = -36 d = 12 x -36 = 4-32 in., then 



B M = -81 x 16000 x 112 3- j = B M = 

81 x 16000 x 10-56 = 136857 in.-lb. The 
actual bending moment to be resisted, how- 
ever, is 600000, and the excess which will 
have to be taken by the compressional steel 
and additional tensional steel equals 600000 
- 136857 = 463143 in.-lb. If the com- 
pressional steel is placed 2 in. from the 
bottom edge, then d c will equal 2 in., and 
the lever arm or distance between the two 
sets of reinforcement will equal d d c = 
12 2 10 in. The additional area of 
steel required in tension will be found by the 
formula : 

463143 



_ Excess B M _ _ 
At = t x (d - d c ) = At ~~ 
463143 



16000 x (12 -2) 
9 sq. in. 



' 160000 , 

This amount, plus -81 in., the area found 
above, will give a total amount of 3-7 in. 
for the steel required in the upper or ten- 
sional area. The steel required in the com- 
pressional area will be found by the formula : 

Excess B M 
Ac == cTx~(3 - d c )' 

The value of c s must be found, and, as before 
stated, this will depend on the position of 
AC in relation to the neutral axis, namely, 

t x (n - d c ) 

; therefore 



c x = 



d n 
16000 x (4-32 - 2) 



37120 
~ Cs = 7-68 



12 - 4-32 

c s = 4833 Ib. 

It will be seen that the steel will be 
stressed at a very low figure. The formula 
for the amount of steel can now be worked 
out as follows : 

463143 

A c = 



A, = 



4833 x (12 ^ 
463143 



= A c = 9-58 sq. in. 



THEORY OF REINFORCED CONCRETE 



in 



Before actually giving the finished section, 
it will be advisable to check the stresses, and 
see that they do not exceed the permissible 
limits, and in order to do this, the position 
of the neutral axis must first be calculated 
by the formula given for double reinforced 
beams, namely : 



/2 m(A t d + A c d c ) 



m (A c 



rm(A c + A t )J 
b 



; therefore 



/J X 15 



X 12 + i)-58 X 2) 



10 

15 (9\5S 4- 37) 



15 (9'5 + 

10 



- v 



30 (44-4 + 19-16) |"15 x 13-2812 

10 10 

15 x 13-28 



10 



/J905-8 I" 199 ' 2 "! 2 _ 199.2 
= V 10 ~_ LjOj ~TO~ 
n = x/190-68 + 396-7064 - 19-92 
n = v/587-38 - 19-92 = w = 24-23 - 19-92 
w = 4-31 in. 

This compares very well with the position 
calculated according to the economical per- 
centage when designing, namely 4-32 in. The 
stress in the concrete can now be found as 
follows : 

2BM 



c = 



bnx 



(n 
d-^ 



c = 



2 X 600000 



10 X 4-31 X (l2 - Ap) + 2 X 9-58 X 15 X ( 4 ^ 2 ) X 12 - 2 

1200000 



c = 



c = 



43-1 x 10-57 + 287-4 x 
1200000 



2-31 



x 10 



1200000 
1995-967 



455-567 + 1540-4 
c = 601 Ib. 

This is quite satisfactory, and t and c s 
can be proceeded with. 



- 601 x 15 x 1-784 = t = 16082 Ib. 
"i =; cm 



c s = 601 x 15 x 



4-3i 



- c, - 4832 Ib. 



All these figures are quite satisfactory, and 
the beam can be considered as being calcu- 
lated for the centre and end sections. 

The diagram in Fig. 128 shows the section 



at the ends of the span. As the amount of 
reinforcement to take the compression at 
the ends was 9-58 in., it will be necessary to 
employ, say, six l|-in. diameter rods, which 
will give a total area of 10-5 sq. in., and these 
must be placed in two rows as shown. The 
depth taken in the calculations will be to 
the centre of the two rows, and conse- 
quently, it will be necessary to allow 3 in. 
of concrete below this point, giving a total 
depth of 15 in. instead of 14 in., which was 
theoretically necessary. This increase will 
have the effect of making the resistance 
slightly in excess of that calculated, and 
therefore it may be adopted. The extra inch 




Fig. 128. Section, at Ends of Span, of Tee 
Beam Fixed at Ends 

of covering is not indicated on the section 
illustrated in Fig. 127, as it is not necessary, 
and the under side of the beam could be 
formed with a very slight camber, which 
would improve rather than detract from the 
appearance of the beam. The question of 
bending up certain bars from the lower sur- 
face to the upper near the supports, and the 
calculations for shearing stress, are not con- 
sidered here. 

TEE BEAMS WITH DOUBLE REINFORCE- 
MENT 

With regard to tee beams with double 
reinforcement, it is not considered necessary 
to give any particular examples for this class 
of calculation, as the same formulae and 
principles will apply as were given for beams 
with double reinforcement when the neutral 
axis falls within the slab or coincides with 
the under side, and, as previously stated, 
these are the most common cases. It must 
be borne in mind, however, that b will repre- 
sent the width of the flange acting with the 
beam in compression. When the axis falls 
below the slab, the beam can be designed 
upon the assumption that the concrete in 
the rib is to be neglected, and the various 



112 



REINFORCED CONCRETE 



formulae previously given can be used, and 
if necessary slight adjustments can be made 
after the preliminary calculations have been 
made. Where double reinforcement is em- 
ployed, the two sets should always be 
connected by binding. 

SHEARING STRESS AND ADHESION 

The question of the shearing stress and 
adhesion has been omitted up to the present 
in order to simplify matters and enable the 
student to become quite familiar with the 
principles of designing for tensional and 
compressional stresses ; but the provision 
of steel in practically all concrete beams to 
resist shear is absolutely necessary, and this 
question has, in the past, received far too 
little attention. When the use of reinforced 
concrete was more or less in its infancy, 
this side of the design was so little dealt 
with that in nearly all cases of tests applied 
to beams where failure occurred, such failure 
was due to insufficient provision against shear 
or diagonal tension, and it was this fact that 
caused the more careful designers to take up 
this part of the subject more seriously. 

The draft regulations of the London 
County Council make provision for shear 
members as follows : " All beams shall be 
provided with adequate shear members, 
and such shear members shall (a) Be 
spaced according to the distribution and 
intensity of the shearing stresses ; but the 
distance from centre to centre of the shear 
reinforcement at any part of the beam shall 
not exceed the effective depth of the beam. 
(6) At least extend from the centre of the 
tensile reinforcement to the centre of pres- 
sure in the concrete under compression. 

(c) Be passed under or round the tensile rein- 
forcement, or be otherwise secured thereto. 

(d) Have a mechanical anchorage at both 
ends, or they shall have a mechanical bond 
with the concrete throughout their length." 

The first point for consideration will be 
that dealing with the intensity and distri- 
bution of the shearing stresses. The shear- 
ing tendency is due to the opposition of the 
weight and the reactions, and as the great- 
est opposition will occur at the supports 
where the total weight meets, as it were, the 
total reaction, it is at this point that the 
greatest shearing stress is found, and the 
stress at any intermediate point will equal 
the reaction at the support, minus any por- 
tion of the load situated between this point 
and the support under consideration. Thus 



the amount of shearing stress is affected by 
the amount and nature of the load, but not 
by the span ; and, cons quently, it is in 
the case of beams which carry heavy loads 
over a short span that the shear becomes 
the most important consideration. The 
shearing stress acts both horizontally and 
vertically, and at any point in the beam 
these two actions will be equal, and as a 
result the shear members are often placed 
at an angle of 45 degrees to resist the 
resultant of the two forces. The distribu- 
tion of the stresses over the area of the 
beam is somewhat curious, as the combined 
action of the bending moment and shearing 
force causes the lines of maximum stress to 
assume a curved form ; and the shearing 
stress will be greatest where the tension 
and compression are nil, namely at the 
neutral axis. 

Diagonal tension is often expressed as 
shear, whereas it is dependent on the shear 
and the longitudinal tension for its inten- 
sity. The actual moment of this intensity 
cannot be ascertained with any certainty, 
and, consequently, it is usual in practice to 
calculate the vertical shear, and take this 
as the measure of the diagonal tension, or, 
at least, provide sufficient steel for this pur- 
pose, when the beam should be efficiently 
designed. The shearing stress, as previously 
stated, is not uniformly distributed over the 
area of the section, but is greatest where the 
longitudinal stresses are least, namely, at 
the neutral axis, and in an ordinary homo- 
geneous beam the stress diminishes from 
the axis to the outer edges, where it becomes 
nil, assuming a parabolic curve, as shown in 
diagram A (Fig. 129). If S equals the total 
shear at a vertical section, then the maxi- 
mum shear which occurs at N A, and is ex- 
pressed by m s, can be found as follows : 
The area of the shaded portion represents 
the total shear on the section = S, and the 
area of this figure = m s x d, therefore 

S 3 S 

| m s x d = S and m s = -7, or m s = ~-j. 

If a diagram is set up as given at A (Fig. 
129), then the actual stress at any point in 
the section can be determined. 

It is unimportant in most cases to know 
the exact distribution of the stress, but 
this instance is given rather to make the 
theory more explicit. 

Now, in the case of a reinforced concrete 
beam, the distribution of the shearing stress 
is somewhat different, and the reason for 



THEORY OF REINFORCED CONCRETE 



this is the fact that the concrete above the 
axis only is called upon to resist longitudinal 
stress, that below the axis being neglected, 
and all the tension is taken as coming upon 
the steel. The result of this assumption is 
that, although the shear above the neutral 
axis will gradually diminish towards the 
outer edge, below this point it will be uni- 
formly distributed over the section as shown 
in diagram B (Fig. 129). As the upper part 
of this figure gives a parabolic curve, the 
equivalent area over which the stress acts 
above the neutral axis will equal two- 
thirds bn, and below the axis 6 x (d - n), 




where b = the width of the section, and s = 
the maximum shear per square inch. In 
dealing with the calculations for shearing 
stress it is also necessary to consider that 
of the adhesion between the steel and the 
concrete, as the two materials can only act 
together when this adhesion is not overcome. 
The permissible allowance for concrete in 
shear has been given in the table of working 
stresses as 60 Ib. per square inch, while the 
allowance for adhesion between steel and 
concrete is given as 100 Ib. per square inch, 
and the shearing stress across the section 
divided by the total circumference of the 
rods or shear members must not exceed this 
amount. If the perimeter or circumference 
of the bars be expressed by 0, then the 
intensity of stress per square inch around the 

S 
bars will be equal to -, -rr~. In the 



--A 



o 



('-9' 



Fig. 129. Determining Stress in Ordinary 
and in Reinforced Concrete Beams 

therefore the total equivalent area, taking 
the shear, will equal 6 x ( d - ^}. Where S 

equals the total shearing stress acting over 
the section, the greatest stress per square 

o 

inch will be equal to 



b X 



and this 



is expressed by the symbol s. 

In designing the shear members, it is neces- 
sary to consider the amount of stress that is 
acting over a portion of the section equal to 
1 in. in depth, and with a width equal to 6, 

S 



and this value will equal 






or & x s, 



designing of shear members, it is usual 
and advisable to utilise stirrups or rods of 
one section in the same beam and vary the 
spacing according to the amount of stress 
to be resisted. As the greatest shear will 
always occur at the abutment, the stirrups 
will be fairly close together here, and will 
gradually increase in spacing towards the 
A point at which they are not required. To 
resist the vertical shear, there are the con- 
crete and the horizontal reinforcement, 
but in the case of the horizontal shear it 
will in most cases be necessary to provide 
vertical members, as mentioned above, or 
crank up some of the tension rods at a suit- 
able point, or provide both, and it will be 
necessary presently to illustrate the method 
of determining the positions of the shear 
members to suit the varying shearing stress. 

To illustrate the use of the formulae already 
given, examples may be taken and their 
application shown. In the case of ordin- 
ary slabs and rectangular beams with single 
reinforcement which are accurately calcu- 
lated to resist the longitudinal stresses, it 
will be found that sufficient concrete is pro- 
vided in the section to withstand the shear- 
ing stress, and, consequently, no special 
shear members are theoretically necessary, 
but for practical reasons they should be pro- 
vided, as they assist in holding the con- 
' crete together, and, furthermore, they will 
be required under the regulations of the 
authority under whose jurisdiction the 
building will come. 

As an illustration, consider a beam sup- 



REINFORCED CONCRETE 



ported at the ends, which is required to carry 
a uniformly distributed load of 3,000 Ib. per 
foot run, including the weight of the beam 
itself, over an effective span of 16 ft. t The 
total load will equal the load per foot run, 
multiplied by the span = 3,000 Ib. x 16 ft. 

w I 
= 48000 Ib. The bending moment = - g - = 



48000 x 16 x 12 



= 1,152,000 in.-lb. Gal- 
<j 

culating the economical size of d by for- 

, 3 /EM. , 3 /1 152000 
mula, we have d =\/ -^=- = d ^/ 

' I I 

= d = V 7 20210 = say 28 in. The breadth 
should equal -6 d, therefore b = -6 x 28 in. 

= 16-8 in., say 17 in. A, = -00675, b d = 
00675 x 17 x 28 = 3-213 sq. in. The 
greatest shear stress, as previously stated, 
will occur at the supports, where it will be 
equal to the reaction. This value will be 

W 48000 
- = 5 = 24000 Ib. The load being 

a a 

uniformly distributed, the shear will diminish 
from the support to the centre of the span, 
where it will be nil, and, consequently, if 
calculations are made for the position where 
the greatest shear occurs, and the beam is 
found to be sufficient, it will be obvious that 
it will be safe at all points. The greatest 
shear is represented by S, and this equals 
24,000 Ib., while the formula for finding the 
maximum stress per square inch has been 
a 

, where s = the 



gven as s = 



b x 



maximum stress per square inch, which must 
not exceed 60 Ib., or shear members will be 
theoretically required. Working this out, we 

24000 24000 

get s = - 



17 x 



24000 



10-08 



\ 



17 x 24-64 



= 57-5 Ib., which is below the per- 

o 

missible limit. The value of n was taken at 
36 d, as the economical percentage of rein- 
forcement was used, therefore n = -36 x 28 
= 10-08 in. as given. 

The adhesion or shearing stress intensity 
round the reinforcing bars will now be con- 
sidered ; it must not exceed 100 Ib. per square 
inch. The amount of steel required was found 
to be 3-213 sq. in., and if bars having a 
diameter of 1 J in. are used, the area of which 
equals -994 sq. in., four of these would give 
an area of -994 x 4 = 3-976 sq. in., which 



is more than" sufficient. To test the adhe- 

S 
sion, the formula -, -^Y must be used. 







- 9 



The circumference of one IJ-in. bar = 3*5343 
sq. in., and the total circumference of the 
four bars = 3-5343 x 4 - 14-1372 sq. in. 
Then the intensity per square inch = 
24000 24000 

W08\ "14-1372 x 24-64 
14-1372 x 

24000 



68-8 Ib., which is well below 



348-3406 
the limit. 

This example should be sufficient to show 
the method of procedure for a simple case, 
and indicate the truth of the statement that 
theoretically no special shear members are 
necessary with a single reinforced beam 
designed with economical proportions. As 
the draft regulations of the London County 
Council state that shear members must be 
provided, and the distance apart, from centre 
to centre, must not exceed the effective 
depth of the beam, we could adopt stirrups 

1 in. wide and T * F in. thick, and place same 
apart at some distance, not exceeding 28 in. 
The span being 16 ft., this can be divided up 
into the requisite number of spaces, as 

192 
follows : 16 x 12 = 192 in., therefore -^ 

= say, seven spaces. If the stirrups are 
arranged to give a space at the centre of the 
span, six stirrups only will be required. The 
next example will be one in which the con- 
crete is insufficient alone to resist the shear, 
and special shear members are therefore 
theoretically necessary. 

As a further example, let it be required to 
design a beam to carry a uniformly dis- 
tributed load of 2 tons per foot run over an 
effective span of 16 ft., such load to be 
inclusive of the weight of the beam itself, 
which is limited in size to 20 in. effective 
depth and 12 in. wide. Total load = 

w I . 

2 x 2240 x 16 = 71680 Ib. B M = -g- if 

beam is supported only, therefore B M = 
71680 x 16 x 12 

8 

size of the beam = 20 in. x 12 in., and the 
economical percentage, if singly reinforced, 
would be 20 x 12 x -00675 = 1-62 sq. in. 
Next find the moment of resistance of such 
a beam, and if necessary provide additional 



THEORY OF REINFORCED CONCRETE 



reinforcement to take the excess bending 
moment, as explained in doubly reinforced 

beams. M R = B M = A t x t x ( d - ^ ), 

and n = -36 x 20 = 7-2 in., therefore B M 

= 1-62 x 16000 x (20 - -^-] - 456192 

in.-lb. 

The actual B M = 1720320 in.-lb., 
therefore excess B M - 1720320 - 456192 
in.-lb. = 1264128 in.-lb., and this must 
be provided for by compressional and 
additional tensional reinforcement. Then 

x Excess B M _1264128 

" 4~~~"/,j~~~.i\ ~ ".( = - CAAn ~ 



A, 1 - 



1264128 



- 4-38 



1 6000 x (20-2) 
sq. in. This 



288000 

amount must be added to the value given 
above of 1-62 sq. in., making a total amount 
for the tensional reinforcement of 6'00 
sq. in. The compressive reinforcement will 
be found by the formula : 

Excess B M t x (n - d c ) 

- , where c = 



x (d - d e y 



, 16000 x (7-2 - 2) 
equals c, = ^ ' = c, = 



d - n 
83200 



A c = 



20 - 7-2 
= 6500 Ib. Therefore, 
1264128 



= Ac = 



1264128 
117000 



6500 x (20 - 2) 
AC = 10-8 sq. in. 

There is no need to explain the method of 
checking the longitudinal stresses, as that 
has already been done in previous examples, 
but the provision for shear may now be 
investigated. The total load is equal to 
71,680 Ib., and the greatest shearing stress 
will occur at the supports, where it will 

, W 71680 

equal -y = ^ = 35840 Ib. The allow- 
ance for shear on the concrete must not ex- 
ceed 60 Ib. per square inch, and therefore the 
value of the section to resist the shear will 
be equal to the equivalent shear area, which 

(n\ 
d " ~o) x 60 Ib., therefore resistance 
/ 

of concrete = 12 



x (20 - 7 J 



x 60 = 

12 x 17-6 x 60 = 12672 Ib. This value 
being less than the actual shear coming upon 
the section, it is obvious that there will be 
an excess equal to 35,840 -- 12,672 Ib. = 
23,168 Ib., which must be met by steel shear 
members. The working stress for steel in 
shear is given as 12,000 Ib. per square inch, 



and the area of steel required will equal 
23168 

V2000 ~ ^ ^ s ^' m * ^ ^ e beam i s SU P" 
ported at the ends only, and the tensional 
reinforcement at the centre of the span 
equals 6-00 sq. in., it will be clear that as 
the bending moment diminishes towards the 
supports the tensional reinforcement may be 
reduced, and consequently one or more of 
the bars may be cranked at a point where 
the shear on the concrete becomes over the 
permissible limit and carried through the 
shear area in such manner that it takes up 




Fig. 130. Vertical Shear in Reinforced 
Concrete Beam carrying Uniformly 
Distributed Load 

the excess shear. Reference to Fig. 130 will 
show how the shearing stress decreases from 
the maximum value of 35,840 Ib. at the sup- 
port to nil at the centre, the diagram illus- 
trating one half of the beam only, and 
consequently the total shear on one half the 
beam is represented by the triangle ABC. 
The shearing value of the concrete in section 
is 12,672 Ib., and if this amount is set down 
to scale from B, as indicated, and a hori- 
zontal line is drawn through to the line A c, 
then the position where it cuts this line, as 
shown by the dotted vertical line, will be 
the point at which the shearing stress 
becomes greater than the concrete alone is 
capable of taking. 

This point may be found by calculation 
as follows : The shear at any point is equal 



n6 



REINFORCED CONCRETE 



to the load at the support minus the load 
situated between that point and the sup- 
port. Now the load at the support = 35,840 
lb., and as the value of the sectional concrete 
to take the shear is only 12,672 lb., it is 
necessary to move outwards from the sup- 
port along the beam until sufficient load has 
been placed between us and the support to 
reduce the load to this amount. It will be 
necessary, therefore, to move outwards 
until an amount of 35,840 12,672 = 
23,168 lb. has been passed over. The load 
is equal to 2 tons, or 4,480 lb. per ft. run, and 
the distance from the support, therefore, 

OQ1 Q 

equals TTOQ = 5 ft. 2 in. If the diagram 



be set out to scale, as in Fig. 130, this can be 
checked. It is from this point that the 
necessary tensional bars can be cranked up, 
and from this point to the support provision 
for horizontal shear must be made. 

On considering a horizontal plane taken 
along the beam at any level below the 
neutral axis, and finding the total stress 
intensity existing along this plane, sufficient 
stirrups can be calculated to take the total 
excess of shear, and by keeping all the 
stirrups of one size, they can be spaced at 
varying distances apart and be thus equally 
stressed. Taking, firstly, the total shear at 
the supports, which equals 35,840 lb., it is 
required to find the shear on a portion 1 in. 
deep and with a width equal to b by 

S 35840 35840 



d 



20 - 



7-2 



17-6 



Of this amount the concrete is capable of 
taking an amount equal to the width of the 
section multiplied by 60 lb., therefore value 
of concrete = 12 x 60 = 720 lb., leaving an 
excess of 2,036-36 - 720 = 1,316-36 lb. Now 
this excess, which has to be taken by the 
steel, will gradually diminish from the sup- 
port towards the centre of the beam until it 
reaches a point 5 ft. 2 in. from the support, 
when it will be nil, as the concrete alone is 
capable of taking all the shear from this 
point to the centre of the beam. This is 
clearly shown in the diagram in Fig. 131. 
Having now obtained the maximum excess 
which requires to be taken by the steel, 
and the length of beam over which this is 
acting, the total excess may be determined 
by taking the mean of the excess at the sup- 
port and at a point 5 ft. 2 in. from the sup- 
port, and multiplying same by the length 



over which it is acting This amount will 
obviously be equal to the area of the triangle 
shown as DBF in Fig. 131, which equals 

DE ; DF _ 1316 ' 3 !! X 6 g = 40807-16 

Zi A 

in.-lb. This is the total horizontal shear, 
therefore, for which steel must be provided, 
and as the allowance for steel in shear is given 
as 12,000 lb. per square inch, the sectional 

. , ... .,40807 
area required will equal? 1 0Ann = 3-4 sq. in. 



If it is decided to use stirrups 1 in. wide'and 
J in. thick the sectional area would equal 
25 sq. in., and as each stirrup will have two 




Fig. 131. Horizontal Shear in Reinforced 
Concrete Beam carrying Uniformly 
Distributed Load 



wings, the area provided by each stirrup will 
equal -5 sq. in. 
The total number of stirrups required 

will therefore equal -^- = say 7, and 

these must be spaced to give a uniform stress 
on each one. This may be done graphically, 
as illustrated in Fig. 131, where a semicircle 
is set up over the length of beam in which 
the stirrups are to be placed, and the length 
is also divided up into fourteen equal divi- 
sions, being twice the number of stirrups 
to be employed. Perpendicular lines are 
then erected to cut the semicircle as shown,, 
beginning at one end and setting up a 
line from the first division mark, and 
then from every alternate mark until the 
opposite end is reached, and the last line 
erected over the point marking the end divi- 



THEORY OF REINFORCED CONCRETE 



117 



sion. The points at which these perpendicu- 
lar lines cut the semicircle are then trans- 
ferred to the horizontal line, marking the 
top of the beam with the varying radii as 
shown, when the positions of the stirrups 
will be obtained. The object of thus spacing 
the stirrups is, of course, to divide up the 
shearing stress represented by the triangle 
DBF into equal portions, and if the lines 
representing the stirrups are continued down 
to cut the triangle as indicated, it will be 
seen that it is divided up into divisions of 
varying width which have more or less 
equal areas. 

To comply with the draft regulations of 
the London County Council it would be 
necessary to introduce the additional stirrups 
shown by dotted lines. These stirrups are 
spaced 20 in. apart, equal to the effective 
depth of the beam, and should have the 
ends turned out to afford a good bond with 
the concrete. In the case of inclined stirrups 
the best angle for these will be 45, and it 
is necessary that they should be firmly 
attached to the horizontal rods if they do 
not actually form a part of same. The 
spacing for these can be found in a similar 
manner to that employed for vertical 
stirrups, except that the inclined line must 
be drawn through the point at which the 
vertical division cuts the neutral axis. 
There is no doubt that the best method is 
that where vertical stirrups are employed 
together 'with a certain number of bent -up 
bars, the method being effective in the 
resistance of diagonal tension. 

COLUMNS 

The theory and calculations in the case 
of columns must be divided into two dis- 
tinct sections, namely, short columns and 
long columns, the reasons for dealing with 
compression members in this manner having 
previously been given in the notes on columns 
in the preceding chapter. In the case of 
reinforced concrete work the column may 
be considered as belonging to the first sec- 
tion where the length does not exceed 
eighteen times the least diameter, and as 
such short columns are considered to fail 
by direct crushing only, the full working 
stress may be adopted, and the design is 
comparatively simple. These will be con- 
sidered in the first instance after a few 
general remarks relating to reinforced con- 
crete as a material for columns. 

The steel is usually placed in the form of 



vertical bars spaced equidistant on the cir- 
cumference in the case of a circular column, 
and at each corner in the case of a rect- 
angular section, except in the case of rect- 
angular sections where the sides are un- 
equal as given below ; and these vertical 
bars must be linked or bound together with 
some form of lateral reinforcement to pre- 
vent the possibility of the load causing the 
bars to burst outwards through the com- 
paratively thin covering of concrete coming 
over them. The steel rods themselves assist 
the concrete core when under the load, and 
prevent the bursting tendency, and to do 
this effectively they must be well connected, 
and by links which are close together, thus 
forming practically a steel cage which confines 



A -i- 



1 1 




I I 


p 


-1 


', ,- L -4 jj 




h 
'l 







Fig. 132. Diagrams Showing need for Lateral 
Reinforcement in Columns 

the concrete and allows it to develop its full 
strength. It will be readily understood that 
in the case of a concrete column with ver- 
tical rods only there will be a great ten- 
dency for the rods to act as shown in an 
exaggerated manner by dotted lines in dia- 
gram A (Fig. 132), whereas, supposing two 
intermediate links were introduced, this 
tendency would be lessened, and the rods 
could only bend as shown in diagram B. 
If sufficient intermediate links are intro- 
duced the tendency will be overcome alto- 
gether, and it will be obvious, therefore, 
that the lateral reinforcement is an impor- 
tant factor in the design. 

The draft regulations of the iondon 
County Council state that the total cross 
sectional area of the vertical reinforcement 
shall not be less than 0-8 per cent, of the 
nrea of the hooped core, and the volume of 
lateral reinforcement shall not be less than 
0-5 per cent, of the volume of the hooped 



n8 



REINFORCED CONCRETE 



core. The effective diameter must be mea- 
sured from the outside of the outermost ver- 
tical reinforcement, and it must be measured 
in the direction of the lateral supports 
which determine the length of the pillar. 

When the column is such that the laterals 
are rectilinear, there must be at least four 
lines of vertical reinforcement throughout 
the entire length, and if the laterals are 
curvilinear, there must be at least six lines 
of vertical reinforcement, and the diameter 
of the rods in any case must not be less than 
in., and the pitch of the laterals must not 
exceed six-tenths of the effective diameter 
of the pier. In the case where rectangular 
piers are used in which the ratio between 
the greater and lesser diameter exceeds 

EEC Jl LINEAR LAjER 



C- f 


A 


(^ 




& 




\'\ 


~ "JTl 


^ 




t 

]HI5 ODTANCf 
EXCEED "A" 

J 




b 


t^ 







Figs. 133 and 134. Rectangular and Circular 
Columns with Respectively Rectilinear 
and Curvilinear Laterals. 

one and a half, the cross section of the 
pillar must be sub-divided by cross ties, 
and the number of vertical rods shall be 
such that the distance between the rods 
along the longer side of the rectangle shall 
not exceed the distance between the rods 
on the shorter side. This is illustrated in 
Fig. 133, where the laterals are rectilinear, 
and the diagram in Fig. 134 gives an illus- 
tration of the use of curvilinear laterals. In 
the case of concrete in columns in simple 
compression the limiting stress is 600 Ib. 
per square inch, and as the steel and con- 
crete are considered as being shortened in 
the same proportion, the stress on the steel 
can never exceed m times the stress on 
the concrete, or, in other words, m x c. 
As m equals 15, then the permissible value 
to allow for the steel equals 600 x 15 = 
9,000 Ib. per square inch. In the case of 
columns which are axially loaded, there- 



fore, and when the height does not exceed 
eighteen times the least diameter, the safe 
load will equal the net sectional area of the 
concrete in the core multiplied by c plus the 
sectional area of the longitudinal reinforce- 
ment multiplied by m x c. As an example, 
let it be required to find the safe load that 
can be carried by a rectangular column 15 ft. 
high which has an effective size of 12 in. 
by 12 in., and is reinforced with four verti- 
cal bars, each having a diameter of 1 in. 

The area of one rod will equal -7854 sq. in., 
and the total area of steel will equal 4 x -7854 

= 3-1416 sq. in. The net area of the con- 
crete will equal (12 x 12) -- 3-1416 = 
144 - 3-1416 = 140-8584 sq. in. The safe 
load on the column will equal A x c + A r 

x m c, where A equals the effective area of 
the concrete, and A v equals the sectional 
area of the steel. Working this out gives 
140-8584 x 600 + 3-1416 x 15 x 600 = 
84515-04 + 28274-4 - 112789-44 Ib., which 
equals just over 50 tons. If preferred, the 
total area of steel and concrete can be taken 
and multiplied by c, and to this the extra 
value of the steel can be added by taking 
the sectional area and multiplying same by 
fourteen times c. Having already con- 
sidered the area of steel as taking 600 Ib. 
per square inch, it will be clear that the 
extra value will equal 9,000 - 600 Ib. = 
8,400, which is the same as fourteen times c. 
Let the previous example be worked out in 
this manner. Then the safe load = 144 sq. 
in. x 600 + 3-1416 x 14 x 600 = 86400 + 
26389-44 = 112789-44 Ib. as before. To 
state this as a formula, then W the weight 

= A x c + A y x 14 c, or A r x 14 c = W 

W - (A x c). 
- (A x c), therefore A v = - , . 

J.4: C 

This last formula will give the method to 
be adopted when designing a short column 
to carry a certain given load, an example 
of which is shown. Design a reinforced con- 
crete circular column which is 12 ft. high to 
carry an axial load of 40 tons. First settle 
the diameter, which must be at least one- 
eighteenth of the height. If we take 9 in., 
this will be one-sixteenth, and therefore well 
within the limit. The sectional area will 
equal 9 x 9 x -7854 = 63-6174 sq. in., and 
the load equals 40 x 2240 = 89,600 Ib. 
89600 - (63-6174 x 600) 



14 x 600 



51429-56 
" = 8400 = = 






THEORY OF REINFORCED CONCRETE 



119 



rods are used, the area of each one will require 
to be 1-02 sq. in. The area of a rod having 
a diameter of lyV i n - equals 1-1075 sq. in., 
and this size will therefore do very well, and 
the total area of the steel will equal 1-1075 x 6 
= 6-645 sq. in. For the purpose of practice, 
check the column and see what safe load it 
will carry, bearing in mind that the practical 
area of the steel equals 6-645 sq. in. as 
against the theoretical area of 6-12 sq. in., and 
the safe load should therefore be slightly 
over 40 tons. 

The safe load equals A x c + A v x m c, 

therefore W = { [(9 x 9 x -7854) - 6-645] 

x 600} + (6-645 x 15 x 600) = W = 

{[63-6174 -- 6-645] x 600 } + (6-645 x 

9000) 

W = 34183-44 + 59805 

W = 93988-44 Ib. - 41-5 tons. 
Which is correct. The column will then be 
as shown in Fig. 134. 

Having dealt with short columns, it now 
remains to deal with those known as long 
columns, namely, where the effective dia- 
meter is less than one-eighteenth of the free 
length. In such columns, as previously 
stated, there is a tendency for the column to 
fail by bending, although, theoretically, there 
should be no such tendency if the column is 
absolutely axially loaded ; but such loading 
cannot be absolutely guaranteed in practice, 
and experiments that have been conducted 
from time to time with long columns go to 
show that it is not advisable to load the 
column to its full capacity as though failure 
would occur through direct crushing. This 
bending tendency cannot be measured or 
calculated unless a certain definite amount 
of eccentricity is known, and consequently 
an empirical formula has to be used where 
certain constants are given deduced by 
experiment. 

It is interesting to note that the London 
County Council draft regulations contain 
conditions which limit the loading on long 
columns, and if the designer is content to 
accept the rules laid down by such regula- 
tions, he will have no need to employ a 
complicated formula. The conditions are 
given, as they will no doubt be of some 
assistance, and particularly in the case of 
checking the carrying capacity of a column 
already designed. Briefly stated, the rules 
are as follows : A pillar shall be deemed 
to have fixed ends when the ends of the 
pillar are sufficiently secured to other parts 



of the construction, having such rigidity 
as will maintain the axis of the pillar at 
the ends in its original vertical position 
under all loads less than the crippling load. 
When both ends are fixed, and when the 
ratio of length to the effective diameter does 
not exceed 

18 The full stress may be allowed on the 

pillars 

21 -8 of above may be allowed 
24 -6 
27 -4 
30 -2 

For other ratios the stress shall be pro- 
portionate to the above. In the case of 
compression members not having both ends 
fixed, the loads shall be as follows : Let P = 
the maximum pressure on pillars and com- 
pression members having fixed ends. 



Condition of Ends 



One end fixed and one end hinged 

Both ends hinged 

One end fixed and the other end 
free and not guided, stayed or 
supported in all directions . . 



Working 

load 



_P 

2 
P 
4 

P 

16 



Unfortunately, there is a great difference 
between the various formulse that have 
been deduced by various authors, and if the 
student were calculating, say, a wooden 
strut, and he adopted Gordon's formulse, and 
checked the result by using Bitter's formula, 
he would find that the latter would give a 
load which would be about five times as 
great as that found by Gordon's. Thus, 
there is no real proof as to what safe load 
can be adopted, or such a variation would 
not be possible, and the most that can be 
done is to recommend the student to adopt 
a fairly safe method. Gordon's formula has 
already been given in the preceding chapter ; 
it is one of the best known in this country, 
and is greatly used. In the absence of exten- 
sive experiments with long, reinforced con- 
crete columns, this formula may be adapted 
for designing such columns, as hereafter ex- 
plained. The formula is as follows : R c = 

Ar 

c r 2 , where R c = the safe load in 

1 + a^ 

the same units as r c , A = the area of the 



120 



REINFORCED CONCRETE 



cross section, r c = the safe resistance of the 
material in compression, a is a constant 
derived by experiment, I is the length in the 
same units as d, and d is the effective dia- 
meter. It will be seen that the value Ar c 
would equal the safe resistance if the column 
were a short one, and consequently the 

I 2 
value 1 + a -ft is the reducing factor which 

is dependent on the ratio of length to dia- 
meter, and also upon a, which is governed 
by the conditions of fixing. In the table 
given on p. 79 it will be seen that for 
wrought-iron and steel, where the section 
was rectangular or circular, the values for 

4 
a were as follow : Ends hinged ^v 



ends fixed 



one hinged and one &- 



fixed jTyyv an< ^ these same constants may be 



taken. As the value A r r in this formula 
equals the safe resistance of a short column, 
this can be substituted by the expression 
which is already deduced for giving this 
value, namely, A x c + A v x me, and if 
the reducing factors given above are applied, 
the complete formula becomes 

A x c + A v x me 

W = /2 

l+(l d* 

and this must be used when checking the 
weight-carrying capacity of a column which 
has a diameter less than one-eighteenth of 
the free length. 

In the case of designing a short column, 
it was shown that the area of steel re- 
quired was calculated by the formula A v = 
W - (A x c) 

j, , where A = the total area of 

the cross section. It will now be obvious 

Z 2 
that the value 1 x a ^ must again be in- 

troduced, and more steel will be required 
than would be the case with a short column. 
The actual weight must therefore be multi- 
plied by this value, giving a formula as 
follows : 



W 



A v = 



(1 2 \ 
1 + a -p ] (A x c) 
/ 



14 c 



Having now deduced formulae for finding the 
area of steel required, and also for checking 
the column to see the safe load which it 
can carry, the application of these will be 



shown in an example. A column is fixed at 
the ends, and has a free length of 20 ft., 
while the load to be carried is 50 tons. The 
column is to be rectangular, and it is limited 
in size to 12 in. square. Calculate the 
amount of vertical reinforcement required, 
and afterwards check the load the column 
as designed would carry. The size given is 
the effective size, and the load is assumed 
to include the weight of the column itself. 
The load equals 50 tons, which is equivalent 
to 112,000 Ib. The effective sectional area 
will equal 12 x 12 = 144 sq. in., and as the 

ends are fixed, the value of a will be 
The steel will be found by the formula : 
W x (l + aj[)- (A x c) 



112000 x 1 + 



14 c 
J_ 

2500 



, therefore 

240 2 \ 
x-Tp-j- (144x600) 

/ 



A, = 

A v - 



14 x 6CO 
112000 x (1 + -16) - 86400 

8400 
129920 - 86400 



= A, = 



43520 

8400 



8400 
A v = 5-18 sq. in. 

If four bars are used, then each will 
require an area of 1-295 sq. in. A round 
bar with a diameter of 1 T ^- in. has an area 
of 1-353 sq. in., and this is the nearest size 
obtainable, and the total area of steel 
required will therefore be 1-353 x 4 = 5-412 
sq. in. As designed, therefore, the column 
is 20 ft. high, fixed at the ends, 12 in. square 
effective size, and reinforced with four 1 T " F 
in. diameter bars. This should next be 
checked by calculating the safe load it will 
carry, and, to be correct, this load must 
not be less than 50 tons. Now 
A xc+A,, x me 



W - 



therefore 



W = 



W - 



138-588 x 600 + 5-412 x 15 x 600 



1 + 



240 2 



25CO 
83152-8 + 48708 



W - 



131860-8 



1-16 1-16 

W = 113673 Ib. = 50-7 tons, which is quite 
satisfactory, the slight amount over 50 tons 
being due to the fact that the actual amount 
of steel equals 5-412 sq. in., which is slightly 
in excess of the theoretical amount calcu- 
lated, namely, 5-18 sq. in. 

In order to form a comparison with the 



THEORY OF REINFORCED CONCRETE 



121 



conditions given in the draft regulations of 
the London County Council, as previously 
stated, it will be interesting to calculate what 
load could be carried by this column if the 
rules given therein are applied. In the first 
instance, it will be necessary to work out the 
load that could safely be put on a short 
column having the same sectional area and 
reinforcement, and then apply the reducing 
factor which is dependent on the ratio of 
diameter to length, as given in the table 
previously. 

The safe load for a short column would be 
found by the formula : 
W = A x c + A v x m c, therefore 
W = 138-588 x 600 + 5412 x 15 x 600 
W = 83152-8 + 48708 = W = 131860-8 Ib. 
This, then, would equal the safe load for a 
short column, and the ratio of diameter to 
length must next be considered. The length 
is given as 20 ft., and the effective diameter 
equals 12 in., and the ratio, therefore, equals 
20. Upon referring to the table, it will be 
seen that where the ratio of diameter and 
length does not exceed 21, the safe load can 
be taken as -8 of that for a short column. 
The safe load for this example will therefore 
equal 131860-8 Ib. x -8, which equals 
105488-64: Ib., or 47-09 tons. This is slightly 
less than the safe load, as calculated by the 
formula, which was 50-7 tons and when 
working in actual practice, it would be neces- 
sary to keep within the limits allowed by the 
London County Council rules, if within the 
County of London. It must be remem- 
bered, also, that the ratio of length to dia- 
meter is actually 20, and not 21 as taken, 
and this, obviously, will affect the result. 
It is very seldom, however, that such a fine 
line would be drawn, and the designer in 
calculating the loads c nnot work so closely 
to the actual loads as to give no margin 
whatever. As stated previously, the vari- 
ous formulae and methods for calculating 
compression members will always be found 
to vary considerably, and in this instance 
there is only a difference of about 6 per cent. 

COLUMNS ECCENTRICALLY LOADED 

It is proposed to give a few notes on the 
eccentric loading of columns and its effect, 
although the value of the eccentricity is 
very difficult to obtain, unless such eccen- 
tricity is caused by certain definite loads 
applied at a certain definite distance from 
the centre of the column. Eccentricity in 
loading is caused in many ways, a common 



case being that where the load from a beam 
is transmitted through a bracket projecting 
from the column, or by the beam not being 
carried right over the column. The eccen- 
tricity is sometimes such that it causes one 
edge of the column to be in tension, and 
sufficient reinforcement must then be pro- 
vided to resist same. The fact of the load 
not being axial will cause the stress in the 
column to vary over the area of the cross 
section, and the compression will be at its 
maximum at that edge which is the nearest to 
the eccentric load, and at a minimum at that 
edge which is the farthest away from same. 
It is necessary, therefore, to consider the 
two extremes, and this can be done by find- 
ing the pressure that would occur if the 
load were axial, and adding or deducting the 
value of the eccentricity. Where the com- 
pression is at a maximum, therefore. 



Max. c = 



W Wy 



A E Z 

where W = the load, A E = the equivalent 
area of the concrete in square inches = the 
sectional area of the concrete + 14 times the 
sectional area of the steel = A + (w - 1) A,,, 
y = the eccentricity of the load in inches, 
or, in other words, the distance from its 
point of application to the neutral axis of 
the section, and Z = the section modulus. 
This last expression was explained in the 
preceding chapter, and the following values 
for reinforced concrete columns are given. 

d 2 
Eectangular Z =^A.d + ^(m- I) A v -4-, 

where A = the total area of the cross section, 
d = the outside diameter of the column, and 
d = the distance from centre to centre of 
the vertical reinforcement. Circular column 
with four bars only 

d 

Zj a" .A. (t "T ?? \)Tl J. ) -i*- ^~, 

d ' 

and for circular column with bars arranged 
in a circle, 

On that edge farthest from the load the mini- 

W W?/ 
mum compression = Mm. c - r- =-. 

From the above formula may be deduced a 
formula for finding W, which is 
Max. c x A E x Z 



W = 



Z + A E x y 



It will now be advisable to work out an 
example to show the application of these 



122 



REINFORCED CONCRETE 



formulae. Find the safe load that can be 
carried by a column 16 in. square reinforced 
with eight IJ-in. rods, the load being applied 
vertically at a distance of 2 in. from the axis, 
and the distance from centre to centre of 




Fig. 135. Designing Column Eccentrically 
Loaded 



rods being 13 in. The maximum stress on 
the concrete is not to exceed 500 Ib. per 
square inch. The plan of the column, as 
given, is shown in Fig. 135, and upon refer- 
ence to this the symbols can be followed. 
The safe load will be found according to the 
formula : 



W = 



Max. c x A x Z 



^ + A E x y 
Max. c is given as 500 Ib., and y = 2 in., 
while A E will equal A + (m - 1) A v = 16 x 
16 + (15 - 1) 9-8176 = 256 + 137-446 = 
A E = 393-445. A v is given by the area of 
the rods, the area of one IJ-in. rod = 1-2272 
sq. in., and the total area = 1-2272 x 8 = 
9-8176 sq. in., as given above. The value 
of Z must next be found by the expression 

Z = ( A x d) + % (m - 1) A v x -i 

therefore, 

Z = (i x 256 x 16) + \ x 14 x 9-8176 x - -- 

Z = 682-66 + (68-7232 x 10-56) 
Z = 682-66 + 725-717 
Z = 1408-377. 

Having determined the value of the vari- 
ous factors, proceed to find the value of the 
weight as follows : 

... 500 x 393-446 x 1408-377 
W = 



W = 



1408-377 + 393-446 x 2 
277060148-571 



As a check to this, and also as an example 
of the use of the formulae for the maximum 
and minimum compression, let it be required 
to calculate the maximum and minimum 
compression on the column above men- 
tioned, and carrying the calculated eccentric 
load. Then the maximum compression, 
which should not exceed 500 Ib., will be 
found by the formula : 

W W y 
Max. c = -j- H . 

The values of A E and Z have already been 
calculated, and therefore, 

126207 126207 x 2 
~ 393-446 4 1408-377 
Max. c = 320 + 179-2 == 499-2 Ib., 

which is within the limit. The minimum 
compression will be given by the expres- 
sion : 



Min. c = 



126207 126207 x 2 



393446 1408-377 
Min. c =320 - 179-2 = 140-8 Ib. 

The edge of the column farthest from the 
load is therefore not in tension, and the 
column is satisfactory. The diagram given 
in Fig. 136 shows how the stress varies across 
the section, and the length of the vertical 
line at any intermediate point represents the 




2195-269 
W = 126207 Ib., or just over 56 tons. 



- 499'2ia 



Fig. 136. Stress in Column Eccentrically 
Loaded 



compression stress at that point. The stress 
on the steel being fifteen times that on the 
concrete the value can be found in any of the 
bars by scaling the length of the line coinci- 
dent with the axis of the bar, and mul- 
tiplying same by 15. 



THEORY OF REINFORCED CONCRETE 



123 



RETAINING WALLS 

It is not proposed in these notes to cover 
the whole theory and design of retaining 
walls, but rather to give some general re- 
marks to enable the student to form an idea 
of the principles governing the design of such 
structures. Walls may be used to retain or 
support either earth or water, and the 
method employed in reinforced concrete is 
very different from that which is customary 
with brickwork masonry or plain concrete. 




Fig. 137. Influence of Angle of Repose 
and Line of Rupture on Design of 
Retaining Walls 

In the last-mentioned case the wall is depen- 
dent on its thickness and weight, which must 
be such that its gravity is not overcome by 
the pressure acting on the back of the wall ; 
whereas in the case of reinforced concrete 
the wall is designed to offer sufficient resist- 
ance by its strength. Tension at the back 
of a reinforced concrete wall is permissible, 
and steel is provided to resist it ; but it is 
obvious that a brick wall, for example, would 
be capable of resisting very little tensional 
stress. A good comparison of the two 
methods of design is offered by the front 
and back retaining walls constructed at the 
Royal Automobile Club (described in a later 



chapter), where both plain and reinforced 
concrete were used. 

In dealing with walls to retain earth, it is 
necessary, in the first instance, to investi- 
gate the nature and value of the force that 
has to be resisted. It will be quite clear 
that a mass of earth standing alone without 
lateral support will not remain with vertical 
faces, but that some of its particles will fall 
past one another, since they have little or 
no cohesion, and form a mound having slop- 
ing surfaces. Sand poured upon a floor forms 
a mound having sloping sides, and it will 
be seen that there is a certain angle with 
the horizontal which the material will main- 
tain even without lateral support. The 
angle depends on the friction between the 
particles, and is called the " angle of repose "; 
it varies in different materials, and also in 
the same material according to- the state of 
consolidation and dryness. The following 
table must be taken merely as a guide, and 
cannot be considered as fixed by any hard- 
and-fast rule : 



Material 



Wet clay or vegetable earth 

Wet sand 

Dry clay, sand, or vegetable earth 

Sandy gravel 

Clean, firm gravel 

Loose shingle 

Hard, dry vegetable earth 

Clay, well drained . . 



16 C 

20 C 
30 C 
26 
45 C 
40 
45 C 
45 



This angle can be considered, then, as the 
natural angle at which the material will 
rest, and this is shown in diagram A (Fig. 
137), where the line of rupture is also indi- 
cated. It is considered that only the wedge- 
shaped portion of earth contained between 
the line of rupture and the face of the wall 
will require to be supported, as upon the 
sudden removal of the wall it is that portion 
which would slip, and this has been proved 
more or less by actual experience. There will 
also be a large amount of friction set up 
between the portion that is tending to slide 
and the portion that is normally at rest, and 
by considering that portion above the line of 
rupture only, an allowance is made for 
this, and it will be seen in the diagram that 
the wedge-shaped portion in tending to slip 
downward is really sliding on the inclined 
plane represented by the line of rupture. 

The line of rupture is found by setting 
out the angle of repose and bi-secting the 



124 



REINFORCED CONCRETE 



angle between this and the back of the wall ; 
the earth that is contained in this triangle 
is calculated and the load worked out 
according to the weight per cubic foot of 
the particular earth under consideration. 
It is usual to take a length of 1 ft. in the 
calculations, both in calculating the weight 
of the soil to be supported and in the design 
of the wall. In the calculations it is neces- 
sary to consider both the vertical portion 
of the wall and also the base, which requires 
to be sufficiently large to distribute the 
pressure over such an area of foundation 
as to bring the load per square foot within 
the safe resistance of the soil. Also the 
base must be of sufficient thickness to allow 
the projecting portions to act as cantilevers. 
The vertical portion will be a cantilever 



on the back of the wall will be found by 
drawing a line parallel to the line of rupture 
from the top of the line representing the 
weight until it cuts the horizontal line drawn 
through D. This horizontal thrust can then 
be multiplied by its distance from the 
point of intersection with the base, which 

will equal -, and this will give the bending 
o 

moment to be resisted. The bending moment 
will gradually diminish towards the top, 
where it will be nil. 

With regard to the base, there are three 
general types of wall employed, as illus- 
trated at A, B, and C (Fig. 138), the selec- 
tion of one of which will depend upon cir- 
cumstances. The type shown at A, for 
example, could not be employed if the 





/////'/ 


V// ///////' 
tAEJH 




'/ \ 






'/ N 






1 


\ fCovnjvs.fOK\ 
1 




\ 


\ 

\ 




y 


\ 




'/, 


\ 




'//, A 


\ 



'/ EAKTO 



B 
Fig. 138. Three General Types of Retaining Walls. 




having a length equal to the height of the 
wall, and it is obvious that the greatest 
stress will occur at the bottom, and this will 
be due to the outward pressure of the earth 
which can be ascertained as shown in dia- 
gram B (Fig. 137), when the centre of pres- 
sure of the triangular prism of earth ABC 

is shown at a point equal to --- from the 

o 

bottom, where h equals the height of the 
wall. The value of the pressure will be 
found by calculating the weight of the earth 
contained in the triangle, and also the centre 
of gravity of the triangle as illustrated. 
The weight will act downward through this 
centre of gravity, and if a line is drawn down- 
ward through this point until it cuts the 
line of rupture at D as shown, and the 
amount of the weight is set up to scale from 
this point, the horizontal equivalent thrust 



adjacent earth was the property of an 
adjoining owner who might raise objection 
to the projection under his land. This was 
the case at the Koyal Automobile Club, and 
it became necessary to design the wall with 
the projecting toe extending into the build- 
ing as illustrated at B. In the former case 
the weight of the earth acting downward 
on to the base will tend to prevent the over- 
turning of the wall, whereas in B this is 
not the case, and sufficient weight must be 
provided to prevent the wall from being 
thrust over on the extreme outer point of 
the toe, which will be the fulcrum of the 
lever. The type shown at C is some- 
what between the others, having a projec- 
tion on both sides. Provision to prevent 
sliding on the base is often made by step- 
ping the under side in such manner that 
vertical surfaces are provided, as shown 



THEORY OF REINFORCED CONCRETE 



125 



by dotted lines in B. The length of the base 
for preliminary calculations is often taken 
as about one-half of the height, and the 
thickness is, of course, calculated from the 
bending moment, which it has to resist as a 
cantilever. 

The type shown at A is often stiffened by 
counterforts at intervals, as shown by dotted 
lines, these serving as stiffeners, both to 
the vertical wall and the base. In London 
work it is often necessary to construct 
vaults under the pavement, when the outer 




Fig. 139. Retaining Wall with Cantilevers 
under Footpath 

wall will act as a retaining wall to support 
the roadway. A type that is sometimes 
employed under such circumstances is illus- 
trated in Fig. 139, where the top horizontal 
portion acts as a cantilever from the vertical 
wall and supports the footpath, while the 
base acts as a cantilever from the bottom 
of the wall and has to resist the earth pres- 
sure. This type of construction was em- 
ployed at Messrs. Whiteleys' new building 
in Queen's Road, Bayswater, London. 

In the case of walls called upon to resist 
the pressure of water, matters are somewhat 
simplified, as there is no variation due to 
weight or angle of repose, and the pressure 
will increase according to the depth of the 
water and always act at right angles to any 



surface. A cubic foot of water weighs 62 lb.^ 
and the pressure in pounds per foot against 
the wall will therefore equal the depth of the 
water in feet multiplied by 62|. It will 
be seen, therefore, that the pressure varies 
directly as the depth as shown by the shaded 
portion in Fig. 140, and the centre of the 
whole pressure will be situated at the centre 
of gravity of the triangle, which will be situ- 
ated at one-third the height from the bottom. 
The value of this pressure will equal the 
height multiplied by the mean pressure, the 

62| x height 
latter being = - - . This pressure 

jt 

multiplied by the height from the intersec- 
tion of the wall with the base will give the 




Fig. 140. Retaining Wall to Resist Water 
Pressure 

maximum bending moment. In the case of 
walls where the surface next the water i& 
sloping, the total pressure will be greater or 
less according to whether the surface is 
sloping away from or to the water ; but 
as the pressure will always act at right 
angles to the surface, the variation in the 
angle of the pressure will compensate for 
any variation in the amount of the pressure. 
The subject of retaining walls is one that 
requires very careful consideration when 
designing, and the student is advised to 
acquire a knowledge of trigonometry before 
attempting to become fully conversant with 
this branch of reinforced concrete work. 
A study of the examples already carried out 
in practice will also be helpful. 



The Erection of a Reinforced 
Concrete Building 



IN tliis chapter will be described tlie method 
or system of carrying out the construction 
of the complete carcass of a reinforced con- 
crete building. Attention will be drawn, 
not only to the correct way of doing the 
work, but also to the faults and pitfalls 
that have to be guarded against. 

In reinforced concrete work there are four 
things that need to be most carefully 
observed : (1) strict adherence to the special- 



and efficiency of the structure depend, and 
if they are not most strictly observed failure 
is almost inevitable. Failure, be it always 
remembered, not only means great and 
unnecessary expense, but involves enormous 
risk to life and limb. It need hardly be 
said that it behoves all those who undertake 
reinforced concrete construction to employ 
only reliable supervision and labour. 



1st engineer's drawings as to the deposition BUILDING A FACTORY 
of the steel reinforcements ; (2) careful and It will be assumed that the building to be 
efficient mixing and punning of the concrete erected is of the warehouse or factory type, 
being some three or four stories in 
1 ^j height (see Fig. 141), and to which 








1 ! 


! . ! 


will be attached the usual engine and 
boiler-house, generally a one-story 
structure. It will also be assumed 
that the site is a bad one, and the 
engineers have found it necessary to 
provide a reinforced concrete pile 
foundation for the main building, and 
a reinforced concrete raft for the en- 
gine and boiler-house. To the ware- 
house there will be a large basement, 
which will involve the erection of a 
retaining wall to support the road- 
way on the 
frontage. 













I ' " 


1 ' 











1 " ' 


' ' 








-dllllLJUN, \ Ti, Carting 







, ! ! 


1 '- !. 


' 


a job of this 
description, 
^ | the first thing 


] 


,^^-fl.->-"m.ilnvii^,""' "4- 

I J 

Fig. 141. Vertical 


1 11 

I 

Cross Section 

( 


mO^. i'-* 1 ^"^ VWWWjdyj 

1 . ( 

of Typical Fac 
Concrete 


rsite of all rub- 
bish, hack up 
all old found 
ations, and 
leave the site 
tory Building in Reinforced clear for im- 
mediate oper- 
ations. Next, 



so that the steel is entirely surrounded by 
this material ; (3) the proper design and 
the complete stability of the falsework ; 
(4) the proper and careful striking of the 
falsework. 

On these items the strength, durability, 



the general foreman must select suitable 
positions for his office and the office of the 
clerk of works, and for the building material 
stores ; for the last -mentioned it will be 
necessary to erect some sheds. The con- 
tractor is now ready to receive materials. 



126 



ERECTION OF A BUILDING 



127 



Receiving the Bars. Considering first 
the arrival of the steel reinforcement rods, 
these should be unloaded carefully so that 
the thinner rods are not twisted ; they are 
easily put out of shape by rough handling. 
All the rods and bars must be sorted and 
bundled in their respective lengths and 
diameters, a complete list of which must 
be kept by the foreman. The sizes should 
be indicated by pegs or in some other way, 
so that the risk of errors may be lessened ; 
the importance of this will be realised when 
it is remembered that a unit of one-sixteenth 
of an inch is adopted in specifying the bars, 
and should bars be in a wrong place or of a 
smaller diameter than detailed trouble may 
be caused. Bars that arrive twisted and dis- 
torted must be put aside to be straightened. 

Steel bars are frequently laid down in the 
open, but this is not good practice, and 
temporary sheds should be erected for their 
reception, because exposed steel becomes 
badly scaled and pitted with rust, which 
ought not on any account to be permitted. 
Though it has been recognised that slight 
rust is beneficial for the protection of steel 
in concrete, deep pitting or rusting means 
the reduction of the diameter appreciably. 
The amount of rust permissible will occur 
while the steel is lying in a temporary 
building. Where there is no room for 
sheds, the steel should be given a coat of 
cement wash and then covered with tar- 
paulins. It is a good practice to erect 
temporary racks or stands in the sheds for 
the reception of the bars, so that they may 
be easily lifted down when required ; this 
method will prevent excessive distortion in 
bars less than f in. in diameter, which get 
very badly twisted when laid in rough heaps 
of considerable weight ; the wholesale 
straightening of bars should be unnecessary. 
At the end of each division in the steel rack 
a board should be attached with the lengths 
and diameters of the rods indicated thereon, 
as in the accompanying table, the numbers 
in the first column indicating the number of 



1 in. 


T V in. 


l|in. 


50 50 
50 56 ! 
35 69 
42 96 
82 116 
95 206 


120 100 
128 126 
141 266 
163 276 
181 300 
200 316 


32 140 
38 166 
58 179 
64 186 
73 190 

80 200 





bars in stock, and those in the second giving 
the lengths in feet and inches. 

By this simple means any bar of any par- 
ticular length can be obtained at once, which 
is impossible when all bars of the same 
length are bundled together irrespective of 
their thickness. 

Aggregate and Sand. The position 
allotted for the deposit of the gravel and 
sand should be as near the mixing stage as 
possible, so as to save long barrow runs. In 
most cases the sand and gravel come to the 
job already screened and washed, and in a 
large contract this is nearly always advisable, 
as otherwise a gang of men would have to 
be kept continually washing and screening. 
Should the contractor be fortunate enough 
to find sand, etc., on the site, the washing, 
etc., will be inevitable. The gravel or 
aggregate must be deposited in a separate 
heap from the sand. Both the clerk of 
works and the foreman should see that there* 
is no clay or loam mixed with the sand or 
gravel. Should broken stone or broken 
brick constitute the aggregate, see that it is 
free from dust ; in the case of furnace slag 
or similar material, no sulphur or other 
impurities should be present. In regard to 
sand, sharp coarse river sand is preferable, 
or good pit sand of various size grains up 
to particles that will pass through a J-in. 
square mesh, and of which at least 75 per 
cent, will pass through a J-in. square mesh. 
Sand must be free from all ligneous (woody), 
organic or loamy substances. 

Cement. For the cement a specially dry 
shed should be erected. The familiar rough 
sheds, which are not proof against weather, 
should not be permitted. As to the testing 
of the cement, that is fully dealt with else- 
where in this book ; but it should be said 
here that the foreman and the clerk of 
works should see that the cement arrives in 
sealed bags bearing the maker's name, and 
they should from time to time make prac- 
tical tests to see that the cement is cool 
and in perfect condition. Cement must 
always be stored out of contact with wind, 
damp air, damp walls, or damp ground, as 
otherwise it quickly loses its strength. 

Water. If practicable, water should be 
supplied from a main, but if not, it should 
come from a good spring or boring. It is 
best to have the water laid on by a supply 
pipe to the mixing stage, and such a supply 
is a necessity when machine mixing 
which is here advocated is adopted. 



123 



REINFORCED CONCRETE 



Mixing Stage. The foreman must con- 
sider carefully the position of the mixing 
stage or the mixing machine, as it is from 
this point that the building grows. Prefer- 
ably the stage should always be open to 




Fig. 142. Mixing Stage 

inspection throughout the entire period of 
the job ; certainly it should not be hidden 
in an obscure corner, where it is difficult to 
watch and check the operations. The ideal 
position for a mixing stage is one where it 
can remain as long as possible and where 
it is convenient for the hoisting of the con- 
crete to the different parts of the building. 



at each end to form handles (see Fig. 142). 
A 2-in. fillet nailed round the board will 
prevent the wastage of cement. A level 
space should be prepared for the mixing 
board. 

Hoisting. Provision should be made for 
hoisting the concrete, timber and steel. 
On a large job a derrick and Scotchman 
should be erected, as well as a concrete 
hoist, some efficient types of which have 
been introduced. 

TOOLS AND APPLIANCES 

Generators' tools include many that 
are familiar to the plasterer, while for 
finishing concrete surfaces he uses hacking 
tools (see p. 244), the idea of which has been 
borrowed from the mason's kit. The tamp- 
ing or punning irons, for ensuring that the 
concrete is well tamped around the reinforce- 
ment, may be of the shapes shown in Figs. 
143 to 146 and 152. Other punning tools, 
special spades, etc., are shown by Figs. 147 
to 151. 




Fi. 143 Fig. 144 Fig. 145 Fig. 146 Fig. 147.-Perforated 

Figs. 143 to 146. Four Patterns of Iron Tamper Spade 



The matter is entirely one for the good 
judgment of the foreman. The mixing 
board, measuring about 15 ft. by 15 ft., 
should be constructed of good, straight, dry 
boards of even thickness, braced together 
at each end, and framed up on scantlings of 
2 in. by 4 in., the longitudinal ones projecting 



For cutting bars, a hack saw preferably 
a machine saw is useful for large sizes ; 
bars can also be severed by nicking and then 
breaking. Hand-operated shearing machines 
are available for rods of ordinary size. 
Small bars used as stirrups can be cut 
through with a hammer and chisel, with a 




IN PILLARS AND BEAMS 

A - the effective area of the pillar. 

Ac - area of compressional reinforcement (in 
sq. in.). 

A E = area equivalent to some given area or area 
of an equivalent section or equivalent 
area. 

As- - cross-sectional area of a vertical or 
diagonal shear member, or group of 
shear members, in the length p, where 
p = pitch of stirrups. 

At -- area of tensile reinforcement (in sq. in.). 

A 7/ - area, of vertical or longitudinal reinforce- 
ment in sq. in. 

a - arm of the resisting moment or lever arm 
(in in.). 

a' = arm ratio = a'd . . a'd = a. 

o c = depth or distance of the centre of com- 
pression from the compressed edge. 

B M = bending moment of the external loads and 

reactions (in Ib. in.). 
Generally, 6 = breadth. 

IN TEE BEAMS 

6 breadth of flange of beam (in in.). 
b r = breadth of rib of T beam (in in.). 

IN BEAMS 
c = compressive stress on the compressed edge 

of the concrete (in Ib. per sq. in.). 
C = total compression on the concrete (in Ib. 

per sq. in.). 

IN PILLARS 

c = working compressive stress on the con- 
crete of the hooped core. 

c s = compressive stress in the steel (in Ib. per 
sq. in.). 

IN CIRCULAR SECTIONS 

Generally, d = diameter. 

IN RECTANGULAR SECTIONS 
Generally, d = depth. 

IN PILLARS 
d = the diameter of the hooped core in in. 

IN BEAMS 

d = effective depth of the beam (in in.). 

d c = distance from compressed edge to centre 

of compressional reinforcement. 
rf.s -= total depth of the slab (in in.). 



IN PILLARS 

d 1 = distance between the centres of vertical 
bars measured perpendicular to the 
neutral axis. 

E,- - elastic modulus of concrete (in Ib. per sq. 
in.). 

E, = elastic modulus of steel (in Ib. per sq. in.). 
I - inertia moment of a member. 
I - length of a pillar or effective length of 
span of beam or slab. 

E s 
TO - modular ratio = vv 

jfic 
M S == maximum shear. 

n = neutral axis depth i.e. depth of neutral 
axis from the extreme compressed edge 
(in in.). 

n' = n/d = the neutral axis ratio . . n'd = n- 

O = perimeter or circumference of bars. 

P = total safe pressure. 

IN PILLARS 

p = the pitch of the laterals in in. (i.e. the 
axial spacing of the laterals). 

IN SHEAR FORMULAE 

p = pitch or distance apart (centre to centre) 
of the shear members or groups of shear 
members (measured horizontally). 

IN BEAMS 

r = A./bd = ratio of area of tensile reinforce- 
ment to the area bd. 

R 1 = left-hand reaction. 

R 2 = right-hand reaction. 

S = total shear in Ib. at a vertical section. 

s = intensity of the shearing stress on con- 
crete in Ib. per sq. in. 

s s = shearing stress on the steel (in units of 
force per unit of area). 

T = total tension in the steel (in Ib.). 

t = tensile stress on the steel (in Ib. per sq. in. ) . 

IN PILLARS 

V = volume of hooped core in cub. in. 

W = total working load or weight on any 

member. 

w weight or load per unit oi length of span. 
y = eccentricity of the load measured from the 

centre of the pillar (in in.). 
Z = section modulus. 



In Gordon's formula: R, ; = total safe resistance to compression. 

to compression. 



total resistance of material 



Note. The above is based on, and modified from, the notation proposed by the Concrete Institute and adopted 
by the Royal Institute of British Architects, but certain symbols have been omitted, since they are not used in the 
book, and a few others have been added. The notation is built up on the principle of an index, the significant 
word in each term having been abbreviated to the initial letter, subscript letters being added in many cases. 
Capital letters indicate moments, areas, volumes, total forces, total loads, etc. Small letters indicate intensity 
of forces, intensity of loads, intensity of stresses, lineal dimensions (lengths, distances), etc. 



ERECTION OF A BUILDING 



129 



hammer and anvil block, as shown in 
Fig. 153, or with cutting pincers or pliers, 
several excellent patterns of which are now 
available. 



bending, especially in the case of bars partly 
embedded in concrete. The claw wrench 
(Fig. 156) is helpful for bending ends of 
stirrups over bars, and is believed to have 




Fig. 148. Fig. 149. Tamper for Producing 
-Ross Spade Fine Surface 



Fig. 150. Special 
Spade for Facing 



Fig. 151. Andrews 
Tamper 



been introduced by the Coignet firm. The 
wrench shown by Fig. 157 answers a similar 
purpose. By means of the key or twister 
shown by Fig. 158, the ends of stirrups can 
be twisted together. For the wiring together 
of the reinforcements where they intersect 
one another, practical workers have their 





Fig. 152. Wooden Tamper 

Bending appliances include hand tools 
and machines. The wrenches shown by 
Figs. 154 and 155 are useful for general 

9 



Fig. 153. Cutting Anvil and Hammer 

own individual methods which they believe 
to be best, but there are undoubted advan- 
tages in employing a simple tool of the type 
introduced in the United States under the 
name of the Curry tyer. The Curry tyer 
is about 12 in. long, and the method of 
using it is shown in Fig 159. The wire ties 
used are a few inches long, the actual length 
depending upon the thickness of the bars, 



130 



REINFORCED CONCRETE 





Fig. 154 



Fig. 155 



Figs. 154 and 155. 

Wrenches for Bending 

Ends of Bars 





Fig. 156 



Fig. 157 



Figs. 156 and 157. 

Wrenches for Bending 

Ends of Stirrups 




Fig. 158. Key or Twister 

for Bending Ends of 

Stirrups 





Fig. 161. Kennedy Bar Bending Machine No. 1 




Fig. 159. Curry Tyer 



Fig. 162. Kennedy Bar Bending Machine, 
Geared Pattern 





a 


D 




6 










D 


D 

-C 


It 


1 


i 


A 






n~n 




--:-"---:::* 


_/i\_ 
B 


* 


"ttn z ' 


D 




1! 






^ 


! 


lf\T 

E ''N;\ 
X C' 






-A 


/c 

.. ^ r 




B 




E x F 

--F--^"-"-^ 






f 


I !( 

1 S \ 


1" 


^ 


, ; 


1 " " m _ 


X A D 


u 






U 






L~^> 





Fig. 160. Elevation and Plan of Bench Bending Machine 



ERECTION OF A BUILDING 



and they have a loop at each end. The tool 
itself is a twisting appliance resembling in 
principle an automatic screwdriver, there 
being a sliding handle working up and down 
in a spiral groove machined on a revolving 
shaft inside the handle. The working end 



of the tool has two hooks ; over one of these 
is passed one loop of a wire tie, the tie is 
then passed around the place of inter- 
section, and the other loop is caught over 
the remaining hook. The sliding handle is 
then drawn back smartly, with the result 



-_- 




Fig. 163. Making Bend to Given Inside 
Measurement 




Fig. 164. Making Bend to Given Outside 
Measurement 




Fig. 165. Making a Double Set 




Fig. 166. Making Sharp Bend in Thin Bar 




132 



REINFORCED CONCRETE 



that the two ends of the .tie are twisted 
together tightly. Ties can be made by this 
means at least twice as fast as when pliers 
and straight wire are used, besides which 
the work is more uniform. 

In all cases it is advisable to bend bars 
cold ; a course that is now easily possible 
with bars up to 1| in. diameter. The small 
bars (up to 1 in. in diameter) can be bent 
on a long, strongly constructed, temporary 
bench to which is attached a vice for holding 
the bar firmly, the end to be bent lying 
between two steel pins fixed in a template 
on the bench ; a strong steel lever with a 
hole in it to fit over one of the pins must be 
used, the bar lying across the lever between 
the steel pin on which the lever rotates and 
the pin fixed on the lever. By pulling the 
lever round, the bar is bent to any angle. 
Fig. 160 shows a machine of this type. To 
obtain the correct angle, a clear chalk line is 
marked on the bench to the angle required, 
and the bar is pulled over to lie on it. The 
machine above described can be made on 
any job at a cost of about 30s., and possibly 
less, as the contractor would probably have 
sufficient stock material at his workshops to 
enable him to fit it up. The bending bench 
shown by Fig. 160 and above referred to is 
based on one illustrated in the Coignet hand- 
book. The rigid bench has a hard-wood top A, 
vice B, and bar rest c. The lever D rotates 
on a fixed pivot E, and has holes for pins or 
rollers F. Such a machine is suitable for 
bars up to 1 J in. diameter. A right and left 
double bar bender on somewhat similar 
lines is shown in the photographic view 
(Fig. 167). 

Another method of bending bars cold is 
by means of a special bending machine, and 
Kennedy's may be mentioned as being a 
most effective device by means of which cold 
bars up to 1 in. in diameter may be bent. 
The Kennedy bar-bending machine is 
made in four patterns. No. 1 (see Fig. 161) 
has a direct lever, and bends bars up to |-in. 
diameter, the smallest radius being \ in. ; 
its weight is 25 Ib. As shown, the bar is 
held between a stop and a central mandrel 
or former, the bending force being applied 
by a lever through a pulley in contact with 
the bar. No. 2 (see Fig. 162) is geared and 
will bend 1-in. bars to a -in. radius ; its 
weight is 124 Ib. No. 3 is much the same as 
No. 2, but in addition there is a ratchet 
arrangement, while No. 4 is a powerful 
worm-geared machine for bending li-in. 



bars. To obtain a given measurement inside 
to inside of bends A B (Fig. 163), having 
made one bend, place the bar as shown, mark 
off distance required, mark the line c D 
square with A B, and then make the second 
bend. When a given measurement E F (Fig. 
164) is to be taken over all, make the first 
bend, mark off desired measurement G 
along the bar, square off from G as before, 
taking care to allow for thickness of the bar, 
and then make the bend. To make a double 
set, having made the first bend, reverse 
the bar and place it against the stop, as in 
Fig. 165 ; let a straightedge be put against 
the mandrel or former, parallel with A, so 
as to indicate the extent of the second bend. 
For making a sharp bend in thin bars, an 
extra former A (Fig. 166), shaped to suit the 
section of the material, is inserted, the bar 
placed between A and the central former B, 
and the lever c, carrying a stop instead of a 
pulley, is pushed round. 

When large diameter bars have to be bent, 
the aid of heat has to be sought, but they 
should not be heated above a cherry red 
that is to say, they should be heated only 
just sufficient for the purpose. For stirrups 
and short lengths of small diameter bars, 
such as T 3 F in. and \ in., a single bend can be 
made quickly simply by bending the bar 
round a steel pin on a bench by means of a 
wrench, as shown in Fig. 168 ; another way 




Fig. 168. Method of Bending Small Bars 

is to fix a piece of steel tubing to a vertical 
wooden post, nail a stop underneath it, place 
the bar in the tube, and pull it over, by 
which means stirrups can be made very 
quickly. 

In almost every case, the bars in a slab or 
wall have their ends just bent over for the 
purpose of forming a key or grip in the con- 
crete ; such bends can be made with a blow 
or two from a hammer. 

DRIVEN PILES 

Now that the contractor has everything on 
the site for the erection of his building, the 



ERECTION OF A BUILDING 



pile making and driving is the first tiling to 
which, he must turn his attention. 

The Pile Skeleton. For the steel skele- 
ton, the correct bars specified must be 
selected and placed on trestles close enough 
to prevent sagging ; the bars are then 
placed in wood templates, which may 
be square or circular according to the 
engineer's requirements. These templates 
must be removed as soon as sufficient bind- 
ing is done to hold the longitudinal bars in 
their correct positions. The next process 
is to add the binding, which may be done 
in several different manners according to 
the design of the pile. In the case of a 
square pile of the Hennebique type, links 
have to be made which wrap round two rods 
at a time, as clearly illustrated in Pig. 169. 
These links must be exactly spaced at the 
given pitch shown in the illustration. For a 
circular pile of the Coignet type (shown 
by Fig. 170) the links or ties encircle the 
bars, and the ends of the links are bent 
tightly round a single bar, and at the point 
where the links touch the bars they should 
be tightly tied with annealed wire. For a 
pile of the Considere type, which is octagonal 
in shape, the binding is of spiral form, the 
spiral being first wound on a drum and then 
threaded on to the bars. This type of pile 
is shown in Fig. 171. The pitch of the spiral 
must be strictly adhered to. Whilst these 
skeleton frameworks are being made, the 
joiners will be preparing the wooden moulds 
for their reception, a detailed description 
of which is given in the next chapter. As 
the moulds can be used and re-used, it is 
at the foreman's discretion how many he 
makes, but there should be sufficient to 
allow of the work continuing without 
hindrance ; the more times one mould can 
be employed the greater the economy, and 
it will here be useful to state that in ordinary 
circumstances pile moulds can be removed 
in four or five days after the concreting. 
When several moulds are completed, the 
steel skeletons, which have been carefully 
lifted from the trestles and laid on planks 
on level ground, should be taken up and 
placed in the moulds, and the cast-iron pile 
shoe will then have to be fixed to the bottom 
of the steel framework in the mould. Having 
made a careful inspection to see that the 
steel framework is lying true in the mould, 
a gang of concreters with punning rods or 
rammers should be set to work ; every bit 
of concrete placed in the mould should be 



carefully punned so that the steel is com- 
pletely covered. Keen supervision should 
be exercised, because if by any chance any 
cavities form, or there is a large piece of 
aggregate loose in a pile, disaster will over- 
take that pile under the first blow of the 
driving-ram. 

In very important work it is better to 
cast the piles in a vertical position, this 
being considered to give a more uniform 
strength over the cross section. 

When the concreting is completed, the 
uppermost face must be carefully levelled 









Fig. 169. 
Hennebique 
Square Pile 



Fig. 170. 

Coignet 
Round Pile 



Fig. 171. 

Considere 

Octagonal Pile 



off, and the pile may be left to dry. In hot 
weather the concrete should be watered (by 
means of an ordinary watering-pot fitted 
with a rose) daily for a week or ten days ; 
but this should not be done until the con- 
creting has been completed for ten or 
twelve hours. Ordinarily, a well-made pile 
can be safely driven in six to seven weeks 
after making. In cold weather the newly- 
made pile must be carefully protected from 
frost by covering with sacking, etc. When 
the piles have been in the moulds for 
four or five days, as already stated, the 
moulds may be struck work that must be 
carefully done, so that the arrises are not 
chipped off and the steel bared. This 




Fig. 172. Lidgerwood Pile-driving Engine and Reinforced Concrete Pile 



134 



ERECTION OF A BUILDING 



applies to circular as well as square and 
other piles, because the " circular " pile has 
two flat sides about 4 in. wide for the pur- 
pose of facilitating the driving, an arris 
being therefore formed at the juncture of 
the flat side and the circular side, as shown 
in Fig. 170. In cases where a wooden dolly 
alone is used without a cast-iron helmet 
on the head of a pile during driving, it is 
advisable when making the pile to use a 
richer mixture for a distance of from 2 ft. 
to 2 ft. 6 in. down from the head, the object 
being to strengthen the head so that it will 
not be seriously damaged under the impact 
(a wooden " dolly " actually takes the blow 
of the ram or monkey). The proportions 
of the concrete for use in piles need to be 
determined scientifically. 

Driving of Piles. Eeinforced concrete 
piles are driven with a special pile-driving 
machine or engine, among the best known 
being the Lecour, Sykes, and Lidgerwood. 
The last-named is shown in Fig. 172. The 
drivers are generally fitted with a two-ton 
ram, which is hoisted by means of a steam 
winch. The ram can be driven at speeds up 
to thirty or forty blows a minute. Care and 
judgment must be exercised to drive with 
steady, uniform blows with a short drop, 
otherwise damage will be done by splintering 
and cracking the pile-head, however good 
and hard the concrete may be. With a 
trustworthy ganger driver who will take 
care piles can be driven without a crack. 
Owing to the great weight of a long pile, 
say one from 30 ft. to 50 ft. in length, special 
arrangements should be made for careful 
handling, hoisting, and placing in position 
for driving. When a pile is hoisted into 
its correct position, it must be plumbed up, 
bolted on or rather through the two 
leaders of the piling frame, and then plumbed 
up again when lowered before any blows 
are given. When a cast-iron helmet is 
used, as generally is the case with a Henne- 
bique pile, a strong cushion of sawdust in 
sackcloth is packed inside the helmet to 
relieve the head of the pile from the severe 
impact caused by the fall of the ram, thus 
saving the head of the pile from shattering. 
When a helmet is not used, an elm block or 
dolly about 3 ft. long is placed on the head 
of the pile for the same purpose as the 
helmet. However, Considere piles, by reason 
of the special form of their reinforcing, are 
generally driven direct, without any helmet 
or dolly. 



Everything being ready, the pile driver 
proceeds with the driving, which is con- 
tinued until the specified set or stopping 
place is reached ; the set is specified in the 
engineer's or architect's specification, and 
must always be strictly observed. The test 
required for the set of a pile usually is, that 
it does not sink more than J in. or J in. 
under ten blows of a two-ton ram having 
a drop of 3 ft. ; but much depends on the 
soil and the nature of the strata through 
which the pile has to penetrate. 

It is desirable to note a few precautions 
that should be taken by the clerk of works 
in testing a pile set. The general practice 
is first to determine the distance of the drop 
of the ram to the head of the pile ; this is 
done by clearly marking on the piling 
frame by means of a chalk line the height 
to which the ram is to be hoisted for each 
blow. For marking the pile, a gauge rod 
is hung from the leaders of the piling frame 
in such a manner that it may swing. A 
line is drawn on the pile at the level reached 
by the bottom of the gauge. The specified 
number of blows is then given, and the pile 
is again marked with a pencil at the bottom 
of the gauge rod. The space made between 
the line drawn on the pile before the delivery 
of the blows and the line drawn afterwards 
determines the going or sinking of the pile 
under the blows. 

In witnessing a test, the clerk of works or 
inspector should observe the following points 
very minutely : (1) When a helmet is used 
for capping a pile, see that the sawdust 
cushion packed inside is the same one that 
has been used during the driving of the pile, 
and that another has not been substituted, 
or that a new one has not been packed into 
the helmet and the old one placed under it 
so that on looking up the inspector sees the 
old cushion apparently undisturbed. Should 
a new cushion be inserted, the pile does not 
get the full benefit of the blow, and conse- 
quently the test is not accurate. 

(2) See that the ram or monkey is raised 
during the test blows to the full height 
marked on the piling frame, and see that 
the men holding the guy ropes do not regu- 
late the fall of the ram so that it loses its 
force just before striking the head of the 
pile. The precaution should be taken of 
checking the length of the gauge rule used 
for measuring the fall of the ram with an 
ordinary rule. It is as well to measure the 
gauge rod every time before it is used. 



136 



REINFORCED CONCRETE 



(3) Watch closely the angle at which the 
pencil is held in marking the lines on the 
pile, as a somewhat cute device which needs 
to be guarded against has come frequently 
under our observation. The pencil was held 
at an acute angle from the bottom of the 
swing gauge, thus marking the pile | in. or 
even J in. above what it would be if the 
pencil were held at right angles to it ; then, 
after driving, the second line was drawn 
with the pencil at right angles to the gauge. 
Obviously, the space between the two lines 
was less than it would be if both of the lines 
had been drawn with the pencil at right 
angles to the bottom of the gauge. An effec- 
tive cure is for the clerk of works to insist 
on marking the pile himself. 

(4) When pile driving is carried out at 
night, see that the pile shoes are not knocked 
off before lowering the pile into position for 
driving. 

Reinforced concrete piles are generally 
placed in groups of two, four, or six. It 
may happen that in driving the first pile 
of the group a set cannot be obtained, and 
the pile is driven right into the ground, even 
then failing to pass the test, always going, 
say, y 1 ^ in. to \ in. more than is permissible. 
Where such an event is likely, the driving 
of the first pile should be stopped, and the 
second one started. The next day, the first 
pile should be given about 120 blows with 
a 3-ft. drop of the ram before applying the 
test, when, in the majority of cases, the pile 
will not go more than -^ in. and probably 
not as much as that. Should, however, the 
pile still continue to go, it must be spliced 
or added to in some other way. The splice 
must be designed by the engineer ; as a 
rule, extra rods are placed at the joint of 
the old pile and the addition. The concrete 
of the pile must be cut away down for some 
distance so that all the reinforcement is 
bared. The new steel for the additional 
length is properly framed up and joined to 
the projecting reinforcement of the pile. 
A wooden mould is placed round the steel 
framework to the height required for the 
additional length, and the whole is now 
ready to be rilled with concrete exactly as 
for a column (described later). When the 
concrete is sufficiently dry, the mould is 
taken off, and five or six weeks later the 
driving may be re-started. 

Cast - in - place Piles. Concrete piles 
cast in place are now frequently used in 
foundation work, one of the best known of 



the type being the " Simplex " (Fig. 173), 
in which system a hollow cylinder is driven 
to a bearing, such as ballast, and then 
gradually withdrawn, the hole left by the 
cylinder being filled in with concrete and 
heavily rammed. This cylinder is made of 
about ^-in. lap-welded steel about 14 in. to 
16 in. in diameter and from 30 ft. to 40 ft. 
long, made up in two or three sections cross- 
welded. The upper portion 
is strengthened by a |-in. riveted 
band about 18 in. deep and a 
narrower band \ in. thick riveted 
to the bottom end just above 
the shoe. The pile is provided 
with a special alligator jaw or 
shoe, which is closed during the 
driving to prevent the entrance 
of the surrounding materials 
into the cylinder or pipe. The 
jaw, which is securely attached 
to the pipe by cable hinges, and 
can be used over and over again, 
is composed of two symmetrical 
wedge parts, which are kept 
closed during the driving by the 
earth pressure. In hard soils a 
detachable cast-iron point is 
often used, it being left in per- 
manently. The cylinder or form 
is driven by a pile driver, which 
is very little different from the 
ordinary type, except that it is 
fitted with a strong pulling de- 
vice attached to the leaders for 
the purpose of withdrawing the 
form after it has been driven. 
When the cylinder has been 
driven to refusal, the driving 
head is hooked to the hammer 
and wire attachment, and both 




s 



Fig. 173.- 

Simplex 

Pile 



are hauled to the top of the leaders during 
the concreting. A batch of concrete of about 
5 cub. ft. is dropped into the form, after 
which the entire form is raised about 1 ft. 
by a pulling device. The concrete is then 
rammed by dropping a heavy rammer into 
the mass. The impact opens the jaws and 
forces the concrete out into the space made 
by withdrawing the form. The process is 
continued until the whole form can be with- 
drawn, and the space it occupied is filled 
with concrete, which sets and hardens, and 
so becomes a concrete pile. This pile is 
sometimes reinforced by bars being inserted 
and so arranged as to allow the rammer for 
the concrete to be dropped down. By the 



ERECTION OF A BUILDING 



use of a wet and sloppy concrete the rammer 
need not be used, as the concrete will settle 
down and surround every part of the rein- 
forcement. The rods inserted should pro- 
ject through the head of the pile for the 
purpose of joining up the beams at their 
junction at the pile head. 

" COMPRESSOL " CONCRETE PILES 

The " Compressol " system is the in- 
vention of a French engineer, S. L. Dulac, and 
its name indicates the compressing of the 
soil. The plant includes three rams which 
are of different shapes and sizes, as shown 
in Fig. 174. The " borer " is of a long conical 





Fig. 174. "Compressol" 
&OE1E.R. Borer, Rammer, and 

Tester 

shape, with a sharp point, the latter having 
a special cavity which brings back a sample 
of the soil reached at each blow, so enabling 
an opinion to be formed as to the necessity 
of carrying the foundation to a greater depth. 
The diameter of this ram at the upper part 
is 2 ft. 4 in. and the length about 6 ft., 
whilst the weight is 32 cwt. The second 
ram is called the " rammer," which has an 
ogival shape with a length of 3 ft. and a 
diameter of 2 ft. 2 in., the weight being 
30 cwt. The third ram is the " tester," and 
weighs exactly 1 ton ; unlike the others, it 
is made to fall with the large base downwards. 
It is shaped somewhat like a frustrum, and 
the greater diameter is 2 ft. 8 in. These 
rams are worked with a frame, in a similar 
manner to that employed for dropping 
the monkey in piling work, with a steam 
winch and boiler ; the frame is, in addition, 
supported on a revolving base plate which, 
in turn, rests on a set of wheels in order to 
allow the frame to be moved longitudinally, 
as well as about its own centre. There are 
practically two systems, one of which is 
used for deep foundations and the other for 
merely a surface compression! of the soil 
when the latter is of a poor nature. 



In forming the deep foundations the 
" borer " is first used, and this is drawn up 
the- frame and allowed to drop on to the 
soil, the height varying according to the 
circumstances, sometimes reaching as much 
as 35 ft. Fig. 175 shows the frame and the 
" borer " in position during the execution 
of the work. An automatic self-acting 
grip, supported by a pulley and chain, is 
employed, and each ram is provided with a 
specially designed rod at the head, as shown 
in Fig. 174, which allows the automatic 
grip to come into action. As soon as the 
ram is dropped, the automatic grip is 
lowered to pick up the ram, and upon the 
latter being wound up to any desired level 
the grip comes into contact with a special 
ring, which is fixed to the frame, and this 
automatically releases the ram and allows 
it to fall ; then the process is repeated. 
A sufficient number of drops is given to 
make a circular hole in the soil ; and in the 
operation all the strata adjoining are com- 
pressed to such an extent that the sides of 
the hole hold up very well in most cases. 

The process of boring and compressing is 
continued until the level is reached at 
which the foundations are to start, and then 
the " borer " is withdrawn and several large 
stones are thrown into the circular shaft 
and subjected to severe blows with the 
" rammer," which forces them outwards 
and spreads the bottom of the hole until it 
forms a wide base, in some cases with a 
diameter of 6 ft. or 7 ft., thus giving a good 
footing for the concrete pier, which is to 
come above. 

When the base is properly prepared the 
process of filling in the hole with concrete is 
proceeded with, and this is accomplished by 
depositing layers about 16 in. to 20 in. thick, 
which are heavily rammed with a few blows 
of the rammer ; cement concrete is used 
where great strength is required. The 
effect of the ramming is to consolidate the 
concrete and give it increased strength ; at 
the same time the surrounding soil is much 
compressed by the spreading out of the 
concrete, which, when completed, has been 
found to measure 6 ft. and more in diameter, 
as against the original size of the hole, which 
was about 3 ft. 

Fig. 176 shows the finished concrete pier, 
and indicates clearly that the object of the 
ramming is admirably obtained. The photo- 
graph shows how the concrete is forced out 
to a greater extent when passing through a 



138 



REINFORCED CONCRETE 



soft stratum, and thereby effecting a greater 
compression at this point, making a more 
uniform resistance at each portion of the 
pier and giving projections which materially 
assist in resisting any downward pressure. 

It is usual, when constructing these piers 
in actual building work, to use large stones 
at the base of the pier and smaller material 
when nearing the top. 



when the ram meets the pier ; that is to say, 
30 tons for a set of 1 ft. If the set is only 
| in. per blow the bearing power will obviously 
be 24 times as great, or, in other words, 
720 tons. Against this, of course, must be 
placed a factor of safety to allow for the 
loss of energy due to vibration, the resist- 
ance of the air, and other causes, when 
testing. 




Fig. 175. " Compressol " Frame and Borer in Ute 



Fig. 176. " Compressol '' Pile 



The bearing power of each pier is ascer^ 
tained by means of the "tester." When 
the pier is nearing completion, the rammer 
is withdrawn and the tester is substituted, 
this being used to give a volley of heavy 
blows on the head of the pier, during which 
time the set is ascertained. Naturally, the 
less the pier is sunk by each blow the greater 
the resistance it will offer when carrying a 
superincumbent load. The weight of the 
ram being exactly 1 ton and the fall being, 
say, 30 ft., a force of 30 ton-ft. is obtained 



When much water is encountered during 
the boring it is necessary to adopt measures 
to prevent it from draining into the well, 
and this is accomplished in a simple manner. 
The hole is rilled with ordinary clay up to 
about 8 in. above the water level, and the 
boring operation started afresh. The clay 
is forced outwards and compressed around 
the well, forming a kind of watertight lining, 
which is usually sufficient to prevent any 
great inflow of water. It may become 
necessary in some cases to repeat the opera- 



ERECTION OF A BUILDING 



139 



tion two or three times, but eventually a 
perfectly watertight well can be obtained. 

In the case of surface compression only 
(which is used where the loads are light, but 
where the soil is " made " ground, or of a 
similar description and needs improvement) 
the " borer " is not employed, but trenches 
are made about 3 ft. deep and the rammer is 
used to make a hole 4 ft. or 5 ft. deep, 
which is then filled with dry stones, bricks, 
or similar material for about one-third of 
the depth, these being subjected to a few 
heavy blows with the rammer, which forces 
them outwards and downwards all round 
into the soil. This process is repeated two 
or three times and the tester is then used to 
give a few blows, after which a great improve- 
ment can be seen in the soil. 

This method is employed in different por- 
tions of the trenches at such distances apart 
as may be necessary under the particular 
conditions of the case, which are governed 
by the nature of the soil and the loads to be 
carried. This is a very cheap and effective 
way of improving the foundations for a light 
structure. 

Pile Gaps. When all the piles have 
been driven and passed, the constructor can 
proceed to cap them, and to form the con- 
necting beams that will support the bottom 
floor. Should the site at the surface be of 
fairly solid substance, no bottom planking 
will be necessary for the pile caps and beams, 
or even for the floor slabs. Fig. 177 is a 
plan of a typical pile cap. 

For the purpose of constructing a concrete 
cap to a pile that has been driven a mould 
in the form of a box is made to surround 



forcements are placed in the beam moulds, 
all the bars properly connected up at their 
intersections over the piles (see Fig. 178), 
and the concreting begun. While the con- 
creting is being done, the spaces between 
the beams may be filled in to the level 





PILE CAP 
2 P/iCS 




Fig. 177. Plan of Typical Pile Caps 
and Connecting Beams 

of the under-side of the slab, by which 
time it is probable that the moulds to the 
pile caps and beams may be struck, taken 
away, and cleaned for re-use. The filling in 
between the beams up to the level of the 
bottom of the floor slab should be completed 
by thoroughly punning and levelling off 
with some good, small, hard material such 
as a mixture of cinders and gravel, clinker, 
etc., so as to form a good bottom on which 
to lay the reinforcement for the floor slab 
(see Fig. 178). 

THE FOUNDATION SLABS 

The placing of the slab reinforcement is 



NOTC - PllE OAgJ sm.'t>KO AfJD 
TAKiU IHTO CAP 




PILES 



Fig. 178. Section showing Pile Caps, Filling, Beam and Slab 



each group of piles. The reinforcement has 
to be placed in each of the boxes, according 
to the engineer's designs. The pile caps are 
connected together by beams which are 
moulded in wooden forms ; the beam rein- 



the next proceeding. The reinforcement 
consists of bars laid to form a meshwork, or 
of a steel mesh formed by welding or " weav- 
ing " fine rods, or by " expanding " steel 
plate. Let it be assumed that ordinary 



140 



REINFORCED CONCRETE 



round bars are to be used, spaced at certain 
distances, and crossed by distributing bars 
wired at the intersections. A practical 
method of placing the main bars in their 
correct positions is to obtain two straight 
pieces of boarding about 4 in. wide, and in 
this to cut notches for the bars to agree with 
the spacing specified. The boards are 
placed, notches upwards, one near each 
beam, and the bars laid to fit the notches 



on the projecting reinforcement, which con- 
sequently is bent anyhow. 

Concreting the Slab. Concreting the 
slab reinforcement already laid is the next 
process, and arrangements should be made, 
if possible, to carry out the concreting 
over one complete area at a time, so as to 
ensure the work being monolithic. There 
are two good practical ways of doing the 
work so as to ensure that the reinforce- 




Fig. 179. Foundation Slab Reinforcement held in Notched Templates 



(see Fig. 179). By this means the bars will 
not get displaced. The notched templates 
can be removed shortly after the concreting 
has begun, quite a small quantity of concrete 
sufficing to hold the rods in place. However, 
before any concreting is done, the cross bars 
or distributing bars have to be laid and 
wired to the longitudinal bars at every (or, 
in most cases, every alternate) intersection. 
The work is not difficult or complicated, but 
it requires great care, and certain precautions 
need to be observed. For example, do not 
allow men to run indiscriminately over the 
steelwork after it has been laid down ; 
instead, form runways with planks packed 
up from the ground so that they clear both 
the meshwork and the beams, it being borne 
in mind that, at this period of the work, the 




Fig. 180. Raft Foundation Beam Reinforcement on 
Wooden Supports 



upper or compression reinforcement and the 
stirrups of the beams are projecting above 
the level of the concrete in the beams ; fore- 
men frequently overlook this, and allow 
boards used as runways to rest by their ends 



ment does not come out on the surface. 
One is to place a round rod, about f in. 
in diameter, under the meshwork, and 
roll it forward as the concreting pro- 
ceeds, this method being best suited for 
use on a suspended floor with flat wood 
sheeting. The method preferred for a 
foundation slab, as in the present case, is 
to place a layer of concrete (say about 1 in. 
thick) from each barrow load, and to lift 
the meshwork through it by means of a 
lifting hook (any piece of steel rod hooked 
at the end). The hook is caught round one 
of the main bars, and lifted about 3 in., the 
mesh being given a slight shake and then 
lowered gently to rest on the thin concrete 
layer. Then the remainder of the concrete 
may be added, well tamping all the time, 
but not too heavily, or the cement will work 
to the top. Punning or tamping needs to 
be done lightly but thoroughly. The usual 
finish is to level the slab with a straightedge 
pressed down on two screeds, and worked 
with a short motion. 

In concreting a floor, the 
concrete should be carried 
forward on a straight line 
across the whole of the width 
of a bay. 

In the concreting of the 
slab at present being considered, those places 
where the columns (over the piles) and the 
walls (resting on beams) will come should 
not be smoothed over, but left quite rough, 
so that good joints can be made later. 




- 

u 



141 



142 



REINFORCED CONCRETE 



The foundation for the main building is 
now complete. 

The raft foundation slab for the engine 
house is constructed in a different manner 
from the foundation for the main building. 



before doing this a strong grout should be 
poured into the mould to strengthen the 
key with the slab concrete. To complete the 
work, the mould must be filled and the con- 
crete carefully punned so that every particle 




Fig. 182. Completed Raft, Kingsway Church, London 



The preliminary step is to level the site 
either by excavating or by making up with 
some good clean material, such as clinker, 
gravel, and ashes. The positions of the 
beams are next carefully set out, and the 
steel skeletons for the beams made in a way 
similar to that described on p. 133 for 
making a pile skeleton. The skeletons are 
placed in their correct positions, and set to 
their proper and exact levels on temporary 
wooden supports, as shown in Fig. 180. The 
slab reinforcement should then be laid in the 
same manner as for the floor just previously 
described, but in this case the concreting to 
the slab must be done 
first instead of to the 
beams, the concrete 
being taken up to the 
side of the beam frame- 
work, so that when it 
set sufficiently to 



of the beam reinforcement is properly 
covered. The moulds may be stuck four or 
five days afterwards. 

Figs. 181 and 182 show the raft for Holy 
Trinity Church, Kingsway, London, which, 
on the completion of the church tower, will 
carry a total weight of 2,500 tons, and which 
is independent of the foundation of the rest 
of the building. A lower slab, 12 in. thick 
and stiffened by small cross-beams each 
2 ft. wide by 2 ft. 9 in. deep, carries two 
main beams each 4 ft. wide by 4 ft. 3 in. 
deep, and the reinforcement is by means of 
patent indented bars. 




s 



Fig. 183. Plan of Retaining Wall with Tapering Counterforts 



bear the weight of men 
and materials the wooden moulds for 
the beams may be erected. As soon as the 
concreting is set, the wooden supports to 
the beam framework must be withdrawn, 
and the framework suspended or, rather, 
steadied by braces across the top of the 
moulds. The moulds having been erected 
and fixed, the beams must be concreted, but 



THE RETAINING WALL 1 

In the erection of a retaining wall sup- 
porting the roadway, the ground must be 
securely shored up and strutted, and pro- 
vision must be made, when erecting the 
shuttering, to strut the ground up from the 
centering, but the shoring must not in the 
least bear on the retaining wall. For the 



ERECTION OF A BUILDING 



purpose of practical illustration, it is assumed 
that the vertical bars have already been 
placed in and secured to the foundation beam 
and slab, which has been specially designed 
for this purpose. These bars must be held 
upright by means of temporary stays, so 
that they do not become twisted. 

The wall would probably be about 6 in. 
thick and plumb on both sides, with a 
horizontal beam half-way up and a capping 
beam at the top. There will also be counter- 
forts which very much resemble a beam in 
a vertical position ; they will probably be 
about 2 ft. 6 in. thick at the base, tapering 
to the width of the capping beam, which 
may be about 18 in. ; moulds will be con- 



structed for these similar to those for a 
column in a wall. The reinforcements will 
be carefully framed up while the concreting 
proceeds as the vertical rods of the counter- 
forts will have already been fixed, as in the 
case of the vertical bars for the wall itself. 
The concreting naturally will be brought up 
with the wall, so that there will be no joint 
between the counterfort and wall. Fig. 183 
shows plan of a retaining wall of this descrip- 
tion. For a wall of this type, it is advisable 
to erect the centering on the external side 
to the whole height of the wall, and when 
this is done the vertical bars may be fixed 
to it at their proper spacing, thus obviating 
the use of temporary stays. The horizontal 




Fig. 184. Reinforcement of Retaining Wall, Royal Insurance Building 



REINFORCED CONCRETE 



r 




Fig. 187. Dia- 
gram showing 
the above 
Retaining Wall 
if Built with 
Brickwork 



Fig. 185 Fig. 186 

Figs. 185 and 186. Reinforced Concrete Retaining Wall for Royal Insurance Building 

bars should next be fixed for part of the way 
up and securely tied at their intersections 
with wire. The wall is now ready for con- 
creting, and the shuttering which should be 
about 3 ft. high should be erected between 
the studs, which are generally spaced 5 ft. 
or 6 ft. apart. The concrete should be 
placed in the wall and carefully punned 
with a wall punning iron, doing the whole 
length of the wall, if possible, in one day ; 
the next day another 3 ft. should be con- 
creted, by which time the lower shutters 
may be struck, cleaned, and used for the 
. third 3 ft., and so on to completion, the 
remaining horizontal bars being fixed as 
the concreting proceeds. When the posi- 
tion for the beam is reached, a two-sided 
mould must be securely bracketed out from 
the vertical studs supporting the shutters. 
The beam reinforcement must be placed in 
the mould and connected with the wall 
reinforcement, the concreting following. 

Scaffolding will have to be erected to com- 
plete the wall, and it should be constructed 
in such a way that it is independent of the 




ERECTION OF A BUILDING 



wall except that it may be tied into the 
studs supporting the shuttering. Care must 
be taken not to shake the wall in erecting 
the scaffolding, because the concrete in the 
wall is still " green." (Concrete that has 
set but is not properly dry is in a condition 
known as " green.") The striking of the 
outside centering should not be done for 
some time, as the wall should be dry right 
through before there is any chance what- 
ever of any weight coming on it. It is better 
to avoid all unnecessary risks. In a few 
cases where the wall is erected close against 
dug-out ground the centering has to remain 
because it cannot be got at, but this is not 
often so, there generally being enough room 
between the outside of the wall and the 
ground for a man to get in. 

A retaining wall at the Royal Insurance 
Building, Lombard Street, London, is illus- 
trated by Figs. 184 to 188, the diagrams 
being comparative and showing stability and 



pressure on ground of reinforced concrete 
and brick walls under identical thrusts. 
The space available did not allow of counter- 
forts. The wall is 26 ft. 6 in. high from 
underside of toe, and is only 21 in. thick 
at the bottom, the thickness at the top 
being 10 in., there being two offsets. The 
wall is vertical at the back, the heel pro- 
jecting for 18 in., as shown in Fig. 186. The 
reinforcement takes the form of vertical 
bars and horizontal stirrups. 

COLUMNS 

While the retaining wall is being built the 
carpenters will be constructing the column 
moulds, and the steel workers will be fixing 
up the columns. Column reinforcement may 
be done in at least two ways : one by erect- 
ing the bars in the mould and putting on the 
ties or binding as the concreting proceeds ; 
the other by framing up the skeleton of the 
column on trestles as before described for 




Fig. 188. A Further View of Retaining Wall, Royal Insurance Building, in Course of Erection 
10 



146 



REINFORCED CONCRETE 



piles. Probably fewer mistakes are liable to 
be made in the second method with regard 
to the exact position of the bars and the 
spacing of the ties, and also the ties will be 
more tightly fixed to the vertical bars. What 
is meant is clearly shown in Fig. 189 be- 
low. When the skeleton columns are made 
they should be carefully stacked in a place 
of safety where the ties will not be disturbed ; 
this precaution is often neglected, with the 
result that the steelwork is trampled on and 
displaced as shown in Fig. 189. The skele- 
ton columns, having been placed in the 
moulds, should be temporarily fixed by 
means of wood templates so that they stand 
truly plumb on all four sides ; and it should 
be seen that the space between the outside 
of the vertical bars and the mould is correct. 
Before any concreting is permitted, the 




CORftKT 



Fig. 189. Cross Sections of Columns showing 
Right and Wrong Methods of Placing 
Reinforcements 

clerk of works should inspect all columns to 
see that there is no dirt, sawdust, or other 
rubbish lying in the bottom, and that a 
good grouting has been poured in to form 
the joint between the floor and column or 
the joint between the column under the floor 
and the column above it, as the case may be. 
As a rule the vertical bars of a column pro- 
ject through the floor some 6 in. or 8 in., so 
that a good connection can be made with 
the column above it. The concreting of a 
column can be carried out in one of two 
ways either by pouring in the concrete from 
the top, or concreting and punning it from 
the bottom, one side of the mould being 
left open for this purpose in the same 
manner as already stated for concreting the 
retaining wall. If pouring is the method 
employed, no one pouring should fill the 
column to a greater height than six times 
its diameter, and less is better. In filling 
columns it is essential that the concrete is 
sufficiently fluid to surround the bars and 
ties, and it should be rammed with a long- 
handled punning iron. Great difficulty is 
caused by the presence of large stones in 
the aggregate, and therefore the aggregate 
used should pass through a |-in. square mesh. 



In the construction of buildings of several 
stories, the diameters of the reinforced con- 
crete columns invariably decrease as the 
building gets higher. The bars of one 
column generally project through the floors, 
as already stated, and they are frequently 
bent in slightly to meet the bars of the 
column above. In one system of making 
the joint, a sleeve piece is fitted to each bar 
of the lower column, and the bottom ends 
of the bars of the upper column enter these 
sleeves. By another system the bars of the 
upper column overlap those of the lower one 
for about 12 in., and the two are bound 
tightly together with annealed steel wire ; 
occasionally the binding is omitted. 

WALLS 

While the columns are going up, the car- 
penters will be erecting the wall shuttering ; 
the walls will probably not be more than 
6 in. thick. The shuttering, centering, and 
concreting for an ordinary straight wall are 
worked in practically the same fashion as 
that described for the retaining wall, except 
that openings will have to be left for win- 
dows and doors, and columns (incorporated 
in the walls) for supporting the beams and 
lintels will be erected practically in the same 
way as described for the counterforts to 
the retaining wall. Extra reinforcement is 
generally placed round the openings ; it may 
consist of a bar about f in. to 1 in. in dia- 
meter, bent to fit the opening with the ends, 
where they meet, overlapping or hooked 
through one another ; and both the vertical 
and horizontal bars in the wall are bent or 
hooked round the bar as shown photo- 
graphically in Fig. 527 (p. 324). 

In fixing the reinforcing bars in the wall, 
close observation should be kept to see that 
the spacing or pitch of all bars agrees with 
the specification and drawings. 

Columns, floors and walls in course of 
erection at the Money Order Office, Hol- 
loway, London, are illustrated by Figs. 193 
and 194 (pp. 148 and 149). 

STAIRCASES 

During the erection of the walls and 
columns provision will have to be made for 
the erection of the staircase, which will 
either be erected in situ or built up with 
separate treads or on reinforced concrete 
stringers constructed for their reception. 
In the present instance it is proposed to 
describe the former system. 



ERECTION OF A BUILDING 



147 



The Factory Act does not allow a straight 
flight to contain more than fifteen treads in 
a single going ; therefore, in most cases, 
when there is a height of from 12 ft. to 14 ft. 
between the floors, two flights must be 
constructed between two floors, connected 
by a half-space landing, with a fireproof 
division wall, as shown in Fig. 190. In a 
staircase of this type, treads are sup- 
ported on a reinforced concrete carriage slab, 
on which they are moulded, the half-space 
landing being formed in the same way as 




CROSS SECTION 




PLAN 



Figs. 190 and 191. Horizontal and Vertical 
Sections through Typical Factory Staircase 

the carriage. The centering for the carriage 
slab and landing must be erected, with 
moulds for beams supporting the carriage 
and landings. The reinforcement is then 
laid in position in the same way as described 
for a floor, and moulds are then made for 
forming the treads and fixed in the right 
position, so that the thickness (say, 4 in.) 
of the slab carriage can be maintained during 
the building of the treads. In some cases 
the treads are reinforced with rods laid in 
longitudinally and hooked into the walls. 



Of course, the walls surrounding the stairs 
must be carried up at the same time as the 
stairs. The concreting is begun at the 
lowest tread and worked upwards, being 
carefully punned the whole time. Stairs of 
this kind are generally finished in granolithic 
about 1 in. thick, a thinner layer invariably 
cracking and peeling off. The granolithic 
for the riser must necessarily be applied to 
the form or mould before the concrete is 
deposited ; that for the treads is done 
afterwards, but the sooner it is done the 
better will be the key between the grano- 
lithic and the concrete. Treads are generally 
finished off by being grooved with a grooving 
roller, so as to prevent their becoming 
smooth and slippery. Figs. 190 and 191 
show a typical cross section through a 
staircase of the type described above. 

INTRODUCING THE REINFORCEMENTS 
INTO BEAM MOULDS 

As already suggested in the case of piles, 
the reinforcement for beams, columns, canti- 
levers, etc., can be introduced in either of 
two ways. One is to build or frame the bars 
up in the moulds, and the other is to frame 
them up first and then to deposit them 
bodily in the moulds. Where members are 
loose and not tied to one another, the rein- 
forcement must necessarily be built up in the 
moulds. Much care and patience must be 
exercised in this part of the work. In 
building a beam, the bottom bars must be 
kept at the proper distance from the bottom 
and sides of the mould, and this can be 
effected by using reinforced concrete blocks 
hollowed out as in Fig. 192 to receive the 



STEEL ROD 



APPROX 



Fig. 192. Hollowed-out Concrete Block to 
Facilitate Spacing of Bars 

bars at the correct spacing, two bars for 
each beam ; the blocks are laid in the 
bottom of the mould, and when the beam 
is concreted up the new concrete will adhere 
to the other concrete. One firm of con- 
tractors always adopts this practice, which 
it has found to be effective, although it 
might give the impression of introducing a 
source of weakness. Another way is to 





'49 



REINFORCED CONCRETE 



place blocks of wood of the right thickness 
in the bottom of the mould, and to lay the 
bars on them ; these, of course, have to be 
removed when there is sufficient concrete 
in the mould to hold the bars in the right 
position ; as there is, of course, a risk of 
the wooden blocks being left in the mould, 
the foreman must watch the work very 
carefully. 

In all cases of building a beam in a mould, 
the stirrups have to be placed in first, and 
their spacing must be correctly marked on 
the centering, each stirrup being adjusted 
after a little concrete has been deposited 
to hold it in place. The use of loose stirrups 
necessitates especial care in this particular, 
since it is easy for a careless workman to 
knock about the stirrups in all directions 
during the process of punning. 



VERTICAL 5UPPORT 
HORIZONTAL SUPPORT 




Fig. 195. Method of Supporting Reinforcement 
in Beam Mould 

A method of supporting reinforcements 
in beam moulds before concreting is shown 
in Fig. 195. 

Difficulty is often met with at the inter- 
section of beams over a column, there being 
trouble in getting the bars into their proper 
positions, and particularly in causing them 
to maintain their proper distance from the 
sides of the moulds. Sometimes the neces- 
sary adjustment is made and made badly 
by the aid of a sledge hammer and a con- 
siderable show of temper ! The most effec- 
tive method is to bend slightly one or two of 
the bars at their ends so enabling them 
to overlap properly. 

In vertical work, very long bars are some- 
times used, and these will always need 
to be properly supported temporarily to 
prevent their being twisted and entangled. 
Long straight bars can safely be bundled 
together and fixed to a suitable support, 
but when the bars have been bent to a par 



ticular form, they must be supported inde- 
pendently. 

For the fixing of stirrups and ties of all 
kinds when framing up a member in its 
mould, their positions must always be 
clearly marked in pencil on the mould itself, 
so that, as the work is brought up, the ties 
can be fixed in their proper places. 

FORMING SKELETON REINFORCEMENTS 

For framing a member, all the bends must 
first be made, the bent bars being then laid 
on trestles, framed up in their proper posi- 
tions on wooden templates, and then tied 
temporarily in their places while the bind- 
ings or stirrups, etc., are being fixed to them. 
In the case of a column bound spirally, the 
bars are placed in wooden templates, in 
which is a central hole through which passes 
a rod to act as a spindle ; the ends of this 
rod are rested on the trestles, so that the 
whole framework can be made to revolve. 
(This, however, does not apply to spiral bind- 
ing as designed for the Considere system, 
for which the binding must be carried out 
on a drum first and threaded on to the bars 
afterwards, as described for the Considere 
pile.) The binding is then securely fixed on 
a bar of the framework, and so wound round 
and round while the framework is made to 
revolve. The pitch of the binding is marked 
clearly on one of the rods. When a length 
of binding is exhausted, the end is turned 
tightly round a bar and another length is 
begun on the bar before that on which the 
first length finishes. The binding is con- 
tinued in a like manner till the column is 
complete, but it needs to be securely tied 
on with annealed wire at every alternate 
bar to obviate slipping. When the binding 
is in the form of rings the work is done in 
the same manner as described for piles (see 
p. 133). 

For framing up a beam, the stirrups are 
cut and bent and then threaded on to the 
beam framework, the bars of which are 
temporarily placed in their correct positions 
on templates, as in the case of a column. 
The spacing of the stirrups is marked on 
one of the bars running the whole length of 
the beam, and the stirrups are tied to the 
main bars with annealed wire accordingly. 
All independent members can be so framed 
up ready for deposition or placing in the 
moulds, and if not used at once they should 
be carefully stacked away in a place where 
they will not be subjected to hard knocks. 



ERECTION OF A BUILDING 



The bending of small bars round larger 
bars is best done with the aid of the wrench 
previously described. 

Unfortunately it is easy for a man with a 
shovel, hammer, or a punning iron to cause 
any amount of mischief if allowed to use 
these tools carelessly. Extreme care should 
be taken in the fixing of reinforcements over 
points of support and also in the making of 
connections. 

MAKING A FLOOR 

The very first thing to be considered in 
the erection of a floor is the centering or 
falsework. Moulds or troughs will be needed 
for the reception of the main and secondary 
beams, and they will need to be securely 
strutted from the floor beneath. They must 
be in true alignment and of sufficient strength 
to bear the weight of the work that will 
come upon them. The wood sheeting is 
laid between the beams to support the 



round rounds was described in connection 
with the foundation slab. For a suspended 
or supported floor, the reinforcing will be 
carried out in the same way, but, of course, 
centering is holding up the floor, whereas a 
foundation slab rests on the ground. The 
reinforcement may take the form of bars 
and rods or of a continuous mesh made with 
small diameter rods or from sheet steel. But 
before any slab reinforcement is laid, the 
beams must be arranged for ; the beam 
reinforcements must either be built up in the 
moulds or put together as frameworks and 
afterwards laid in them. Wooden blocks 
should be laid across the mould to support 
the top or compression bar of the beam ; 
and wooden strips should be inserted on 
each side to maintain the correct distance 
between the reinforcement and the faces 
of the form ; but great care must be taken 
that all these blocks and strips are removed 
as soon as there is sufficient concrete in the 





%3fa?^t'?t:%:rt^ 

' " " ' ^ ' ' " 



Fig. 196. Typical Floor with Continuous Mesh Reinforcement 



weight of the floor slab and of the men 
working on it. Descriptions of the various 
methods of constructing floor centering will 
be found in the next chapter. 

Before any steel reinforcement is placed 
in or on the floor centering, all the moulds 
for beams must be thoroughly cleared of all 
shavings, sawdust, dirt, or other rubbish 
which always accumulates during the erec- 
tion of the centering. 

For the purpose of preventing concrete 
adhering to the centering, it is advisable to 
close the pores of the wood by thoroughly 
wetting all internal surfaces just previous 
to depositing the concrete. When a better 
finish is required, the boards may be advan- 
tageously limewashed or treated with soft 
soap before the steel is placed in position or 
the concreting started ; this makes the 
centering easier to remove and gives the 
work an improved appearance. 

The reinforcing of a floor with ordinary 



moulds to keep the bars in place. Fig. 196 
shows a beam mould with the reinforcement 
adjusted by the means just described. 

Concreting must not be started until all 
the reinforcement is complete and correct. 
Obviously, mistakes cannot be rectified once 
the concrete has been introduced. 

The concreting is next carried out in 
exactly the same way as described for the 
foundation slab beams, the concrete being 
taken up to the level of the top of the moulds 
only, and left as rough and as rugged as 
possible, so as to make a good key for the 
slab concrete, which is added after the slab 
reinforcement has been laid. 

Continuous wire mesh is generally sup- 
plied in large rolls containing from 150 ft. 
to 300 ft. run, and varying from 3 ft. to 
6 ft. in width. For laying this to form a 
floor reinforcement, the rolls are hoisted 
on to the centering and placed side by side 
at one end. The roll is then undone and 



152 



REINFORCED CONCRETE 



the end taken into the wall (see B, Fig. 196) 
and fixed with staples to the centering, the 
mesh being next unrolled over the entire 
length ; temporary blocks are placed on 
the beams (see A), over which it passes to 
bring up the mesh to the level required over 
the supports, as shown in the engineer's 
drawings. The correct length having been 
cut off, the free end of the mesh is fixed 
into the wall (see B). The other rolls are 
laid in the same way, side by side, the space 
between them being determined by the pitch 
of the main or tension strands of the mesh ; 
for example, if the pitch of the mesh is 
4 in. the selvedge of one roll is 4 in. from 
the selvedge of the other roll. Reinforce- 
ment of this type can be quickly laid, since 
there is no spacing and tying of rods or bars. 
When sheet mesh instead of the wire 
mesh is used, first of all it is laid flat on 



The concreting of suspended floors is 
done in the same manner as described for 
the foundation slab in the earlier part of 
this chapter ; but it may be well to give in 
this place some further particulars of the 
process of concreting as applied to floor con- 
struction. The concrete should not be so 
thin that the water may run, and carry with 
it the cement, through any openings and 
cracks in the centering, thus causing the 
slab when dry to have a honeycomb struc- 
ture. The more water used in mixing 
concrete, the greater is the shrinkage, but 
workmen are more likely to make con- 
crete over-wet than over-dry because of the 
greater ease in handling and the less punning 
that will be required. The concrete should 
be sufficiently moist to be worked properly 
into the moulds and round the reinforce- 
ment. 



1-2 F SPAtS I 

_|^g^jfj^^ - 



Fig. 197. Typical Floor with Sheet Mesh Reinforcement 



the centering. The sheets may butt over the 
supports, for here there is an additional sheet 
laid near the top of the slab, these additional 
sheets extending over '2 (one-fifth) of the 
span on each side of the centre of the sup- 
port. Fig. 197 shows a cross section of 
this type of floor, and clearly illustrates 
what is meant by the above. Other mesh- 
reinforced floors are illustrated in a later 
chapter devoted to the various commercial 
systems. 

In laying an expanded metal floor, the 
sheets must be so laid on the centering that 
the diagonal strands in the sheets all slope 
in one direction, otherwise the ends of the 
sheets will not properly key into one another 
where they overlap. If the sheets bulge 
slightly when laid in position, a few nails or 
staples should be driven into the centering 
to hold the metal down ; when the center- 
ing is struck, the staples, etc., will remain in 
the concrete, but can easily be clipped off. 
As already stated, while the concreting of 
this type of floor is proceeding, the mesh 
should be lifted and shaken when there is 
a layer of concrete f in. or 1 in. thick on 
which it can rest. 



As far as is practicable, the concreting of 
a floor slab over a complete area should pro- 
ceed at one time so as to ensure the work 
being monolithic ; but when a whole floor 
cannot be completed in one day the work 
should be divided as follows : For a main 
beam, the concreting must be carried to the 
centre of a column. A secondary beam must 
be filled up to, and at the same time as, the 
main beam to which it is connected. In a 
slab, the concrete must be carried to the 
centre of a beam. A column must always be 
carried to its full height. Walls should be 
worked along a complete length, and any 
stoppage of work should occur at a horizontal 
level. 

Concrete spread on a floor should be carried 
forward on an even line across the whole 
width of a bay. 

It will be readily understood that the 
remainder of the floors, columns, and walls 
in the factory will be carried up to the flat 
roof, which is exactly the same as a floor, 
except that there may be openings left in 
it for lantern lights, that the concrete, in- 
stead of being laid level, will be laid to 
falls, that gutters will be hollowed out to 



ERECTION OF A BUILDING 



take the water, and that a parapet will be 
erected in continuation of the walls. 

THE ERECTION OF CENTERING 

From what has already been said, it will 
be seen that as the work proceeds all the 
centering will have to be struck, cleaned, 
and, if possible, re-used work in which the 
foreman will need all his ingenuity to effect 
legitimate economy, which is a different 
thing from scamping. The foreman must 
arrange that the centering is always erected 
in advance well in time for the placing of 
the reinforcement. He should strike the 
centering in plenty of time for it to be pro- 
perly scraped and cleaned before refraining 
it, but everything he does must be consistent 
with absolute safety. In the case of floors 
of large areas, the centering for the height 
of and including two floors should be erected 
as soon as possible, so that by the time half 
of the second floor concreting is done, the 
centering from the first floor can be struck 
and used for the next ; but he must exer- 
cise extreme caution to see that the bottom 
planks of the beam forms and their supports 
are not removed until it is safe to do so. 
All slabs need to be temporarily supported 
after the removal of the sheeting. The 
sanction of the architect, engineer, or clerk 
of works should always be obtained before 
any striking is begun. 

The ease with which centering is removed 
is a good criterion of the quality of its con- 
struction ; but, however well it has been put 
together, it is necessary to employ a number 
of careful men to strike it. At least five 
or six should be detailed off for this work, 
and it should be definitely explained to 
them that the concrete is not yet thoroughly 
set, and that they must exercise care to 
prevent the jarring or chipping off of arrises, 
etc. 

Opinions vary as to the period that should 
elapse before the false-work is removed ; 
but the following is safe practice. 

Floor slabs and beams. The centering may 
be loosened in seven or eight days and 
struck in ten, except the bottom planks 
and supports to beams. 

Wall panels. The strutting can be re- 
moved in two or three days in moderate 
weather ; the sooner they are struck, the 
better chance the wall has of drying out and 
hardening. 

Columns, etc. Three to five days under 
similar conditions to the above. 



It is as well to loosen all centering before 
striking, in order to prevent the concrete 
from sticking to the boards ; moreover, the 
air circulating through will harden the sur- 
faces of the concrete, which will not be so 
liable to be chipped. 

All suspended members must be left 
supported by temporary shores placed at 
judiciously chosen points, and left for any- 
thing from fifteen to thirty days. Where 
support is most needed is at the centres of 
beams and floor slabs. On no account must 
any weight be allowed on any floor or 
against any wall for at least three weeks 
after the removal of the centering, except 
that of the timber for the floor above and 
of the men worldng on it. 

FINISHING CONCRETE SURFACES 

The finishing of concrete is considered in 
detail on pp. 241 to 255, but it is desirable 
in this chapter to complete the discussion 
of the practical work of erecting a reinforced 
concrete building by giving a few notes on 
the subject. Concrete walls can be cleaned 
down, all excrescences removed, and hollows 
filled in, and then whitewashed this, of 
course, applying only to buildings of the 
warehouse type. They can be decorated 
with plaster in the same way as any other 
building, but for this purpose the concrete 
ought to be hacked or scored to form a 
key for the plaster. In this hacking, great 
care must be exercised, because a careless 
man with a bush hammer or similar imple- 
ment can do much damage, although this 
may not be visible to the eye. The hacking 
must be done while the concrete is green 
that is to say, friable. Take the case of a 
reinforced concrete wall 4 in. thick that is 
to be plastered inside and finished in stucco 
outside, necessitating hacking on two sides. 
It is obvious that a man giving the green 
concrete wall hard knocks with a hammer 
will be causing infinite mischief by dis- 
integrating the concrete, and thus destroy- 
ing its adhesive strength and monolithic 
nature. 

THE FIXING OF WINDOW FRAMES, 
DOOR FRAMES, ETC. 

Provision always has to be made for the 
fixing of frames, etc., to the openings left 
for them ; an effective method is to insert 
screws in their proper positions in the con- 
crete, with their worms bound round with 
wire, and when the concrete is sufficiently 



154 



REINFORCED CONCRETE 



set, to give each screw a turn ; it will be 
found that the wire is a perfect hold for 
the screw, and the frames, etc., can be fixed 
without difficulty. 

The Craig screw-bore (see Fig. 198) is a 
great convenience when it is desired to attach 

fittings, wood 
finishes, etc., to 
the concrete. 
The method of 
using it is first to 
insert a master 
bolt, as illus- 
trated, through 
a hole made in 
the side of the 
form ; the wire 
screw - bore is 
Fig. 198. Craig Screw-bore then screwed on 

to the bolt, the 

reinforcement placed in position, and the 
ends of the screw-bore wound around the 
reinforcement if this is thought necessary. 
Before removing the forms, the bolt is in- 
screwed, thus leaving the screw-bore cor- 
rectly placed, and, of course, most rigidly 
held, and forming a fixed nut into which a 
screw threaded the same as the master 
bolt is easily inserted. 





STEEL CORNER BARS 

Steel kerbs or corner bars built into con- 
crete steps answer three purposes : as regards 
appearance they give a finish to the con- 
struction, they provide a durable edge at 
the very place where durability is most 
needed, and to a great extent they act 
as reinforcement. United States firms who 
have specialised in these kerbs apparently 
agree that the form shown in Fig. 199 is the 
best for the purpose, the illustration showing 
the Wainwright kerb, which is made of such 
a shape as to be well anchored into the con- 
crete. The bars are made in various sizes 
for various widths of steps. The appearance 
of a staircase fitted with such kerbs is shown 
in Fig. 200. An interesting application of 
the same idea is shown in Fig. 201, which 
is a section through a reinforced concrete 
column, the four edges of which are built up 
with corner bars made of mild steel and 
well galvanised. In the illustration, A in- 
dicates the corner bars and B spreaders made 
of stamped steel 1 in. wide and | in. thick, 
which maintain the corner bars in proper 
position while the concreting proceeds. 
Should additional reinforcement be required, 
it may take the form of upright bars, c, 
with wire links, D. 



Fig. 202. Ebco 
Corner Bar 




Fig. 199. The Wain- 
wright Steel Kerb 





Fig. 200. Steps with Steel Kerbs 

. 

A method that is more generally employed 
is to insert breeze bricks at various places 
round the opening, so that the frames can 
be screwed into them. 



Fig. 201. Section through Column with Steel 
Corner Bars 

Another type of corner bar the Ebco 
also provides a rounded corner, but is made 
with anchors spaced approximately 17 in. 
apart, as shown in Fig. 202. 



Forms and Centerings 



Design. Owing to the fact that the con- 
struction of the forms, or shuttering and 
centering, or horsing accounts for a very 
large proportion of the total cost of rein- 
forced concrete work, economy in the false- 
work is most desirable, and the only proper 
way of ensuring this is by the adoption of a 
correct design in the first place. This is 
recognised in the United States, for the 
scheme of the centering is usually decided 
in the office before the work is begun. Good 
design in centering is evidenced by stability 
and rigidity, ease of removal or striking, and 
facility of rapid re-erection. In the case of 
a building which is regular in form and con- 
tains a number of stories which are alike 
except as regards dimensions, thickness of 
floor and scantlings of beams and columns, 
a well-designed centering may be used 
many times before the roof is reached. In 
the case of members of unusual shape, it is 
only by means of proper design that excessive 
waste is avoided. 

Centering should be designed in such a 
way that it may be struck readily without 
jarring or hacking the arrises off the con- 
crete, and to attain this object as few nails 
as possible should be used, thus necessitating 
close supervision, for the carpenter never 
loses an opportunity of driving a nail. Beam 
and column moulds and wall panelling should 
be so made that the various members fit 
into one another without nailing, the whole 
being made secure by bolting, wedging, and 
like means. 

The contractor is tempted to employ too 
light timber, with consequent risk to the 
stability, but, on the other hand, too heavy 
timber is unnecessarily expensive and 
awkward to handle. The timber used should 
be strong enough to take, without appreci- 
able deflection, any load imposed upon it by 
the reinforced concrete, the men working 
on it, and the tamping. Where practicable, 
the scantlings of the timbers to be used will 
be given in this chapter. 

It is essential to maintain the perfect 
alignment of beams, and to see that all 
vertical members are truly plumb, for 
reasons that are obvious. Further, all 



moulds should be carefully and sufficiently 
strutted so as to prevent sagging or bulging 
after the concrete has been deposited ; and 
all joints should be made tight enough to 
prevent the thinner part of the concrete 
running through, but at the same time allow- 
ances must be made so that when the boards 
swell with the moisture they can give but 
not bulge. 

It is advisable to add angle fillets to beam 
and column moulds so as to form a chamfer 
on the concrete, (a) thus obviating sharp 
arrises which tend to knock off when remov- 
ing the centering and so reducing the amount 
of patching up to be done afterwards ; 
(6) making spalling less easy in case of fire ; 
and (c) giving a finished appearance to the 
work. It is a good plan to construct all beam 
moulds with a camber at the centre of at 
least \ in. ; the beam will not come out 
straight on the removal of the centering, 
but in any case, the camber looks better 
than a slight bulge in the middle, and it is 
well known that ancient Greek architects 
cambered all horizontal lines to make them 
look straight to the eye. 

Failures may often be quite as much 
attributable to weak false-work or to the 
removal of the false-work before the con- 
crete is sufficiently set, as to such a cause 
as bad design. Since any settlement that 
may take place when the concrete is newly 
made may easily prove disastrous, rigidity 
of the forms becomes essential. 

The forms and centerings described in this 
chapter are taken from actual examples in 
practice, this course being considered prefer- 
able to the discussion of generalities. 

SIZES OF TIMBER 

Beams. Bottom plank, 2 in. to 2f in. 
thick. Sides or cheeks, 1J in. for secondary 
beams and If in. to 2 in. for main beams. 

Columns or 'pillars. Sides, If in. to 2 in. ; 
corner studs, 4 in. by 3 in. and 5 in. by 
2 in. 

Slab. 1-in. boarding, maximum spacing 
for 1-in. boarding, 2 ft. ; maximum spacing 
for If-in. boarding, 4 ft. ; maximum spacing 
for 2-in. boarding, 5 ft. 



156 



REINFORCED CONCRETE 



Slab beams. 6 in. by 3 in., spaced 2 ft. to 
2 ft. 6 in. apart, according to nature of slab. 

Walls. Kunners, 9 in. by 3 in. 

Mr. Sandford E. Thompson gives the 
following safe loads for timber struts in 
forms for floor construction : 



Strut 
Length 


3" x 4" 


4" x 4" 


6" x 6" 


8"x8" 




Ib. 


Ib. 


Ib. 


Ib. 


14 ft. 


500 


700 


900 


1100 


12 ft. 


600 


800 


1000 


1200 


10 ft. 


700 


900 


1100 


1200 


8 ft. 


850 


1050 


1200 


1200 


6 ft. 


1000 


1200 


1200 


1200 



The following tables have been carefully 
worked out and will serve as a useful guide : 



bridges ; (9) place new posts under girders near 
beams with cross heads running along girder 
bottoms in same position as original post ; 
(10) prop up slab joists temporarily as near 
beams as possible; (11) draw nails holding 
beam sides to beam bottom ; (12) lower wedges 
under posts supporting beams and turn cross 
heads lengthwise and wedge up again ; 
(13) remove beam sides ; (14) remove girder 
sides ; (15) remove slab sheeting and joists. 

SETTING OUT 

The correct position of the forms can result 
only from extremely careful setting out on 
a proper system allowing of the main lines 
being referred to during the progress of the 
work. " Horses " are made with 3-in. by 
2-in. stakes driven into the ground, with 
floor boards, from 6 in. to 9 in. wide, and any 
convenient length up to a few feet, nailed to 



TABLE SHOWING DISTANCES BETWEEN SUPPORTS TO BOARDS, &c., FOR FALSE-WORK 



Boards, 
Planks, &c. 


1" 


U" 


11" 


If" 


2" 


2i" 


2J" 


3" 


Distances 


2'0" 


2' 6" 


3'0" 


3f /> n 
6 


4' 0" 


4' 6" 


5'0" 


5'0"-6'0" 



TABLE SHOWING DISTANCES APART FOR VARIOUS SCANTLINGS IN STRUTTING 
AND SHORING TO FALSE-WORK 



Scantling 

(varies with 
height) 


4" x 2" 


4J" x 3" 


6" X 2" 


7" X 2" 


8" x 2J" 


9" x 3" 


10" X 5" 


12" x 6" 


Distances 


2'0" 


3'0"-3'6" 


2'6"-3'0" 


3'0"-3'6" 


4'0" 


4'6"-5'0" 


5'6"-6'0" 


7'0"-8'0" 



TABLE SHOWING DISTANCES APART FOR DIE SQUARE TIMBER FOR FALSE-WORK 



Scantling 


5" x 5" 


6" X 6" 


7" X 7" 


8" X 8" 


9" x 9" 


Distances 


4'0" 


4'6"-5'0" 


5'6"-6'0" 


6'0"-8'0" 


8'0"-10'0" 



ORDER OF STRIKING THE FORMS 

The order of striking forms is thus sum- 
marised by Prof. Johnson : (1) Wedges in 
column form struck and placed in sacks for 
hoisting to next floor ; (2) clearance pieces ; 

(3) sides of column form under girder ; 

(4) keys at beam sides ; (5) sides of column 
form under beam ; (6) post up beam as close 
to girder as possible with new post ; (7) re- 
move posts under girder bridge ; (8) remove 



their tops, as shown in Figs. 203 and 204 ; 
and on these horses the significant lines are 
accurately marked with a knife. For a 
building with square angles (see Fig. 203), 
one main line having been determined and 
marked on horses, a line is set out at right 
angles, by means of a builder's square, on 
another row of horses, and the operation is 
repeated until the necessary rectangle is 
formed. Carefully check the dimensions 



FORMS AND CENTERINGS 



157 



from those figured in the drawings, and do 
not trust to scaled dimensions. See that 
opposite sides agree in length, and check the 
accuracy of the right angles by measuring 



Fig. 204. 

Horse used in 

Setting Out 




lines, the lines of foundations, etc., may now 
be marked as in Fig. 203. Acute and obtuse 
angles are set out as in Figs. 205 and 206 ; 
in such cases the accuracy of the diagonal 
dimension is of the utmost importance. 

FORMS FOR PILES 

For a square pile the mould required 
is very simple ; it is constructed with three 
sides as in Fig. 207, the uppermost side 
being left open for inserting the steel skeleton 
and concrete. Angle fillets should be fitted. 
The mould must lie perfectly level, and the 
skeleton must be securely suspended in the 
correct position and be dead level, so that 



Fig. 203. Setting out 
Piers of Rectangular 
Building 



Fig. 206. Setting 
out Obtuse Angle 
of Building 

30 in. one way and 40 in. the other way, the 
diagonal between the two points then meas- 
uring 50 in., which equals the square root of 
the square of 30 plus that of 40. The outside 
lines of the walls and columns, the centre 



Fig. 205. Setting 
out Acute Angle 
of Building 



it may be entirely surrounded by concrete. 
The ends of the rods must be properly placed 
in the cast-iron shoe at the end of the mould. 
An octagonal pile is constructed as 
above described, with the exception that 



158 



REINFORCED CONCRETE 



large fillets are fixed to the inside of the 
boxes. 

For a round pile a mould is constructed 
with two circular sides or cheeks, made with 
narrow battens after the fashion of a barrel 
and bound with iron straps ; the mould must 
give the pile two flat sides, so that they can 
lie flat up against the guides of the piling 
frame. In forming this mould (see Fig. 208) 
a straight plank is used, and through it 
bolts are passed about 2 ft. apart for the 
purpose of holding down the straps that 
bind the circular sides. This plank is laid 
down dead level on sleepers. The skeleton 
frame the reinforcement is next laid on 
this plank, and then the circular sides or 
cheeks are attached and the straps bolted 
at the top, as shown. The open space 

JfAYS A&OUT 2-4 " PfTCH 

4- . 



joists and rebated flooring on which to make 
and cure a few piles. Figs. 210 and 211 show 
the method. The side shuttering of the 




I 



Fig. 208. Bolted Form for Round Pile 

piles is formed of 2-in. by 8-in. verticals held 
together by 1-in. by 7-in. cleats bid on 
rebated flooring. The sides of the shutter- 
ing are kept in position by 2-in. by 4-in. 
distance pieces, the whole being kept in 
position by struts and 1-in. by 7-in. boards 
temporarily tacked to the top. 

A method of American origin is shown 

in Figs. 212 to 214 ; the parts dissociate as 

shown, and, for curing, the green piles are 

placed on 2-in. by 12-in. 

) planks resting on beams 

' ' > 3 ft. 6 in. apart. 



FOUNDATION FORMS 



Fig. 207. Form for Square Pile 



at the top of the mould is left for the con- 
creting, and is the same width as the 
bottom flat plank. 

The same shape can be 
made by using wood tem- 
plates of the shape shown 
in Fig. 209, and fixing the 
narrow battens to them. 
The two sides are held 
together by nailing short 
stays on the top side, as 
illustrated. 

As in the case of a square 
pile, the skeleton frame- 
work is suspended by hooks 
to the top stays to keep it 
in correct position, and the 
hooks are removed when the concrete is 
half-way up the mould, so that there is no 
chance of the steel framework being dis- 
turbed from its correct position. 

Most contractors set up a platform of 



In preparing the site 
for a foundation slab, 
the ground must be brought to a true level 
and boarding placed about the footing if the 
nature of the soil demands it. If ribbed 

foundations are 
STAYS A&OVT 24 ' PfTCfr 




TEMPLATE 



used they must 
b e excavated 
for and suit- 
able boxing 
provided. The 
concrete is 
poured into the 



Fig. 209. Stayed Form for Round Pile 



forms in layers and the reinforcements added 
at the proper levels. Fig. 215 shows how the 
foundation forms are fixed up when the slab 
has had sufficient time to set hard enough to 
bear the weight of the men and materials. 



FORMS AND CENTERINGS 



The moulds for the beams are very readily side of the moulds about 2 ft. 6 in. apart, 
constructed, it being only necessary to form as shown dotted in the illustration. They 
the two sides. When these sides are fitted up can be withdrawn when the sides of the 



h E - - 
I 4 * 1 - 

r* : 






- 


- 12. CONCRETE. 


"" 


"vVALL 










Fig. 210 

/DISTANCE PIECE ^ 



FORM 



R3RM 



y l/IOIHPH-L rlCl^C -^ 


~l" 




^ > 
PILE 

X xrf 


1 


M 




^ %; 
HLL 


1 


IE 


i 


^ ^ 

PILL 

ik A 




sV 


ij c{ 


{ 4 if 


/ i! tf ij 



ZHZ PLANki 



WALLPLATE 



z 



Fig. 211 ( FOLPINC WEPGES 

Figs. 210 and 211. Elevation and Enlarged Cross Section of 
Pile-making Platform 




-2*12. t~2*a 

Fig. 212. Plan and Ele- 
vation of American 
Pile-making Forms 
and Platform 



Fig. 213. Enlarged Cross Section through 
Form shown in Fig. 212 



Fig. 214. Concrete Piles 
and Forms Dissociated 



in their proper positions they must be firmly 
stayed and strutted to each other, both top 
and bottom as shown. By carrying the top 
stays over the width of the end beams and 
strutting the off-side of these to long pegs 




Fig. 215. Foundation Form Built after Slab is Hard 



driven into the ground (if it is firm enough 
to hold them), the mould will be secured 
without breaking away. If this method of 
strutting from pegs is impossible, the moulds 
may be held by bolts passing through each 



moulds are struck. Fig. 216 is a photo- 
graph showing forms for pile caps, etc. 

The ordinary box mould for foundations 
is simply four sections of the required depth, 
two of the sides extending beyond the 

others, all bat- 
tened and 
bolted together 
(see Fig. 217). 
The 2-in. by 
2-in. arris fillet 
and the 4-in. 
by 2-in. batten 
on the ex- 
tended side 
form a groove 
for the bat- 
tened section 
to slide into. The 4-in. by l|-in. battens 
are a 4-in. by 3-in. batten once sawn. The 
square-headed f -in. rods have a minus thread, 
and 6-in. by 6-in. by J-in. square plates act 
as washers. By alternating the fixed square 




Fig. 217. Typical Box Form for Foundation 




Fig. 216. Forms for Pile Caps, Foundations, etc. 



1 60 



FORMS AND CENTERINGS 



161 



head of rod and nut the form is made 
capable of easy adjustment to bring it 
square and can be readily removed and 
altered. 
In the type of form shown by Fig. 218, 




by 4-in. battens to serve as stop pieces. 
The four sides having been placed in posi- 
tion on the rough base and nailed together 
through the stop pieces, 4-in. by 4-in. posts 
were skew-nailed on the outside as illus- 
trated, and holes bored for the passage of 
annealed iron wires, which connected two 
facing posts, exactly as shown, and which 
were tightened by twisting with a rod. The 
external wires were cut when 
6>"* fa' &OAKOS the time came to remove 
the forms, the projecting 
ends being hammered in and 
finished ofi with 
3*2 &KACL a trowel. It is 
advisable to intro- 
duce strainers to 
take the stress 
caused by tighten- 
ing up the wires, 
and to prevent the 
form being pulled 
out of shape. 



Fig. 218. Cheaper Type 
of Form for Founda- 
tions 



the 3-in. by 2-in. braces keep the mould 
rectangular and also serve as handles. This 
is a cheap and effective type of 
form which can be recommended for 
repetition work. 

Form Strengthened by Twisted 
Wire. In the United States, found- 
ation and other forms are often held 
together by means of twisted annealed 
wire, all bolted rods and shoring 
being dispensed with. While English 
engineers might fear that die-square 
results could not be obtained by such 
a method, it must be admitted that 
it is cheap, simple, adaptable to a 
wide range of applications, and de- 
serves to be more extensively em- 
ployed. Fig. 219 shows such a 
form, the detail at the foot of the 
figure illustrating the type of found- 
ation square tapering piers arched 
together for which the particular 
form shown was required. The 
sides consisted of 1-in. boarding 
nailed to 2-in. by 4-in. battens. 
Two opposite sides were 14 in. wider 
than the pier, and to the edges of 
their inner sides were spiked 2-in 
11 




Fig. 219. Form 
Strengthened 
witb Wire 




Fig. 220. Typical Column 
Form with SHd-in Front 
Boards 



162 



FORMS AND CENTERINGS 



163 



COLUMN, BEAM, AND SLAB FORMS 

The most frequently used forms in rein- 
forced concrete are those for columns, beams 
and floor slabs, so that any economy effected 



whole fomfsides at once with vertical board- 
ing held in position by battens, this method 
necessitating tha >pouring in of the concrete 
from the top, and consequently the use of 




Fig. 221. Typical Column Form with Spiked-on Front Boards 



in their design enabling their re-use without 
waste is sure to tell favourably in the cost 
of the structure. 

Three Methods of Constructing 
Column Forms. There are three methods 
of setting up column forms. 

The first is to construct the forms for the 



a wet mixture, because ramming would 
probably displace the reinforcements. 

The second method is to build up the 
whole four sides gradually with horizontal 
boards held in position by battens at the 
angles, and brought up in short sections as 
the work proceeds. 



/i 



&&OCT 




Fig. 222. Clamped 
Form for Short 
Columns 



164 




Fig. 223. Cheap Type 
of Column Form 



165 



i66 



REINFORCED CONCRETE 



The third method is the most economical, 
and therefore the one most usually adopted. 
It consists in erecting only three sides in 
position at first, formed of vertical boards ; 
the fourth side is gradually brought up with 



as the work proceeds, in short lengths placed 
between a guide and the angle fillet on each 
side. The other portion of the mould is 
made with the boards vertically battened 
and distance-pieced and bolted into position. 






&QAGDS 



3*2 




Fig. 224. Column Form with Two Sides held between Fillets and Battens 



horizontal boards as the concrete is applied, 
thus permitting thorough supervision of the 
work as it progresses. 

The edges of the columns are usually 
chamfered by placing small angular slips 
in the corners of the forms. 

Fig. 220 shows a form constructed with 
three sides only, the fourth being built up 



In Fig. 221, the form is built up in board- 
ing, 9 in. by'^lj in., all horizontal, strongly 
nailed to 4-in. by 2-in. uprights. To make 
the front A, the boards are spiked on as the 
work proceeds. The back is held together 
with 3-in. by 2-in.' battens to allow of 
quicker re-use. The angle fillets are lightly 
spiked in place. 



FORMS AND CENTERINGS 



167 



In the type shown by Fig. 222, the 
boarding is vertical on all four sides and 
held together with 4-in. by l|-in. battens 
strongly spiked on. In addition to the 
ordinary spiking, wooden clamps, out of 
3-in. by 2-in. stuff, and bolted together at 
the angles, keep the form in shape ; they 
rest on the horizontal battens. This method 
of construction is employed only for the 
shorter height of pillars,|_and has the dis- 



arranged at intervals according to the 
height of the pillar. The life of this type of 
form is not long, because of the spiking on 
of the front boards. 

In the method shown by Fig. 224, the 
9-in. by If-in. boards are all horizontal and 
are battened together on sides A, B, and c. 
Side D is inserted as the work rises, as short 
lengths of If-in. board. In addition to 
spikes, the form is bolted through, as shown, 




Fig 225. Column Form with Two Sides held between Fillets and Battens 



advantage .of not allowing easy inspection 
or close punning, and of causing derange- 
ment of the reinforcement in the process of 
depositing the concrete in the mould. 
Spikes, or nails, clenched at the angles, are 
sometimes used instead "of -bolts. 

Fig. 223 illustrates a cheap type of form in 
which the boards, placed on as the work 
proceeds, are simply spiked or strongly 
nailed on to the built-up three sides ; 6-in. 
by IJ-in. and 8-in. by IJ-in. boards are 
held together'with 4-in. by 2-in. battens on 
each of the three sides, with -in. bolt rods 



and sometimes a strainer is inserted to take 
the pressure of the bolts. 

Fig. 225 shows a case in which 6-in. by 
2-in. vertical planks are battened together 
on three sides of theTJorm and upright 
members, r 3 in. by 2 in., are spiked on the 
sides so as to make with the angle fillets 
two sets of grooves to receive the back and 
front, the latter being slipped in from the 
top as the work proceeds. As illustrated, 
the batten on the back is clear of the bolt. 

The forms for beams are made in a 
number of ways, the best being to have two 



i68 



REINFORCED CONCRETE 



sides and a base easily detachable (see Fig. 
226). The 7-in. by 2-in. planks are joined 
together by 4-in. by 2-in. battens to form 
the base, while 9-in. by IJ-in. planed boards, 
with 4-in. by 2-in. battens form the sides. 



with 2-in. planks battened together with 
4-in. by 2-in. stuff, the battens to the base 
being extended and strutted as shown with 
3-in. by 2-in. struts. A half-round mould 
is inserted in the angles to give a rounded 




Temporary ties, say 4 in. by \\ in., are 
nailed across the top before filling in and 
punning. The battening is arranged at 
3 ft. 6 in. apart. An effective finish to 
the'arris of the beam is obtained by insert- 
ing"^ hollow mould (see Fig. 228) instead 
of the angle fillet. 



Fig. 226. Typical 
Beam Form 



finish to the beams. This is, how- 
ever, a wasteful design, and is not 
adjustable to further uses. Fig. 231 
shows a form for producing splayed 
angles between pillar and beam. 
The system of false-work shown by'Fig. 
230 illustrates the junction of two beams, 
and also the floor centering in -position. 
The dead shores are arranged at intervals. 
At the edges the IJ-in. boarding is rounded 
off as shown to give a good finish to the angles 
of floor and beam. The method of striking 



r 




/77 





Fig. 227. -Boards with Splayed Edges to allow for Expansion 



In using dry, sound timber, a provision 
to allow for swelling is made by running 
the stuff through a mill arranged to splay 
one edge, as shown in Fig. 227. It is advis- 
able not to run it to a feather edge, but to 
allow, say, a T V m - fillet. 

Fig. 228 shows a beam form constructed 



the floor centering is to withdraw the 4-in. 
by 2-in. batten, which is put in in conveni- 
ent sections ; drop slightly the 9-in. by 
3-in. joists, and then take out the boarding. 
Immediately support the floor with head 
and cill piece for a further pericd. This 
design is open to many objections : splayed 



Fig. 228. Beam Form 
Strutted from Ex- 
tended Base Battens 




J*0 PtATL 



Fig. 229. Folding Wedges 
under Dead Shore 



169 



iyo 



REINFORCED CONCRETE 



fillet is better than tlie hollow moulding ; 
the side boards had better be horizontal; 
and the joists should not rest on the beam 
sides but be propped independently, so that 
the sides, of the beam can be removed first. 
The centering for floor slabs is built up 
with a series of deals from beam to beam, 
supported by head pieces, dead shores, and 
cills on the lower floor at frequent intervals. 



wedges. Fig. 229 shows the arrangement at 
the base of a 6-in. by 6-in. dead shore, but 
it is preferable to have the wedges at the 
top of the upright. 

Various Beam and Column Forms 
Described. Fig. 232 illustrates the simple 
Hennebique type of beam form. A is the 
bottom board, 2 in. thick, the exact width 
of the beam, and supported by posts F. 




Fig. 230. Form for Two 
Intersecting Beams 



See that the shoring is taken to a firm 
bearing that is, carried right through all 
the floors as in Fig. 246 (p. 176) not only to 
support the weight of the floor being con- 
structed, but to ensure that damage is not 
done to the floors already in yet still in a 
somewhat green condition. The system of 
wedging should be arranged in such a way 
that it can be eased at given periods, ensur- 
ing the beam taking an even setting and 
bearing. The cills, and often the heads, 
are provided with folding, hard-wood 



At the top of the side boards B are nailed 
battens or cleats to carry the ends of the 
secondary beams D, also supported by 
posts F. The upper boards E of the main 
beam forms are then placed in position, 
together with the side boards H of the 
secondary beam form. Finally, the lagging 
or floor slab centering is placed on the 
top of the side boards of the beams. The 
whole is held together with iron clamps, as 
shown in Fig. 233. At the bottom of each 
strut there should be hard-wood folding 





Fig. 232. Two Cross Sections through Henne- 
bique Floor and Plan of one Bay 



R 




CL/MP 



Fig. 233. Beam Form held by Clamp 



BOLT 



SCRBff 



Fig. 234. Column Form with 
Bolts and Thumb-screws 




Fig. 235 



7*2 

UPRIGHT 



WEDGES 



Fig. 236 



Figs. 235 and 236. Beam Forms used in Messrs. Sainsbury's Premises, London 



172 



FORMS AND CENTERINGS 



173 



wedges, useful in setting and striking the inserting wedges as shown. A section 
moulds, and these should be adjusted until through the beam form is shown in Fig. 235. 
all is perfectly firm. IE the filling in of the The point to be specially noted is the rebated 



Fig. 238. Plan of Column 
Form 



SEDUCING 
FIUE.T- 




Fig. 237 Beam and Column 

Forms used at a Bermondsey 

Warehouse 



Fig 239. Plan Showing Posi- 
tion of Clamps for Reduced 
Column Form 

concrete has taken the camber, previously 
allowed, out of the beam, the wedges should 
at once be driven home, so that the camber 
is again obtained. 

The beam and column forms adopted at 
Messrs. Sainsbury's pre- 
mises in Stamford- Street, 
London, S.E., are shown 
in Pigs. 234 to 236. The 
column forms of 1^-in. ver- 
tical planed boards were 
built up on three sides 
complete. A shutter, made 
in two heights, of IJ-in. 
boards with cross battens 
was used for the fourth 
side. The whole was 
clamped together by 2-in. 
by 5-in. clamps 1 ft. 6 in. 
apart, adjusted at the corners with bolts 
and thumb-screws ; the angles of the 
columns were ovolo moulded by the insertion 
of corner fillets as in Fig. 234. The columns 
can be reduced inside the same clamps by 



support to the beam bottom. By means of 
fillets of various sizes placed in these rebates, 
the beam can be made of less breadth or 
depth, and so save cutting up the cheeks of 
the beam mould. Fig. 236 shows how the 




Fig. 240. Isometric 

Sketch of Forms for 

a complete Bay of a 

Warehouse Floor 



beam form may be reduced in depth and 
width without destroying any material. 

Figs. 237 and 239 show the beam and 
column forms used at Peak, Frean and 
Co.'s warehouse in Bermondsey. The beam 




174 




Fig. 242. Beam and Column Forms used at H.M.'s Stationery Office 



IB. 



D//////y/ 



:i J^'BOO- 


-< 






i" o" * 


/ 

/ 


< p 


/ 






S>'*3* 







We. 








-f 


71 














m 










B--I 


. . . . . 


_ 




~ 




-: _ 


m 










t- 


ttft- 




t 




^p 


*v 


Kf- 

























* P05T3 




Fig. 243 Fig. 245 . 

Figs. 243 to 245. Adjustable Beam Forms designed by H. Kempton Dyson 



175 






176 



FORMS AND CENTERINGS 



177 



forms are supported by 2-in. by 4-in. uprights 
braced by cross battens nailed on. The top 
cross brace at the under-side of the beam 
is bolted to the uprights, and it supports 
the bottom board of the beams. With 
reference to the column forms, should it be 
desired to reduce the size of the column, 
the lower pair of clamps is brought closer 
together and the upper pair fitted to 
the grooves in the lower pair and wedged 
in the new position. The column can be 
reduced in the other direction by inserting 




Fig. 247. Form for Twelve-sided Column 



3 /& DOLTS 



FLUTE nOULDS 
OF PLASTER OF PAfUS 




Fig. 248. Form for Fluted Column 

fillets of the proper size between the clamps 
and the boarding as shown. The main beam 
forms are supported on 2-in. by 4-in. up- 
rights in pairs at every 3 ft. 6 in. of the 
length. The top cross pieces immediately 
under and supporting the bottom board of 
the beam are bolted to the uprights. The 
secondary beams rest on fillets nailed to the 
sides of the main beams and also on 2-in. 
by 4-in. uprights at 3-ft. 6-in. centres. The 
lagging of the floor slab is supported on 2-in. 
by 4-in. joists 2 ft. apart, the ends of the 
joists resting on the secondary beam forms. 
12 



Fig. 240 gives a good idea of the usual 
construction of the centering of floor slab, 
girders and secondary beams ; the illus- 
tration shows a complete bay of a warehouse 
floor in isometric projection. 

Beam and Column Forms in H.M.'s 
Stationery Office. Fig. 242 shows the 
beam and column forms for the Government 
Stationery Office in Stamford Street, S.E. 




i 
i 




. 


TTTTI ' 


( 1 1 1 1 U 


-t 














ui' 11 ' * 


. " 





Fig. 249. Form for Diminished Column 

The main beams have a span of 21 ft. 3 in., 
and the intermediate beams 15 ft 2 in. The 
columns are 20 in. by 20 in. There are so 
many bays of exactly similar dimensions that 
the centering could be used over and over 
again. The drawings explain themselves, 
but a point to be noted is the method of 
supporting the boarding for splaying the 
haunches of the beam on 2-in. by 4-in. 
fillets nailed to the column. A photograph 
showing much of the false- work is reproduced 
in Fig. 241 ; the steel gantry, which is a 
conspicuous feature in this view^ was 




3 
33 
c 

I 




U 



178 




179 



i8o 



REINFORCED CONCRETE 



employed for raising the building 
materials, electric travelling cranes 
running upon it. 

Beam Forms Designed by H. 
Kempton Dyson. Figs. 243 to 245 
show beam forms designed by H. 
Kempton Dyson. A point to be 
noted is that the beam can be re- 
duced in depth by means of a 
blocking piece. The cleats on the 
uprights can be readjusted to any 
height, thus saving the cutting of 
the uprights to different lengths 
to vary the height. A different 
size of base-plate at the bottom of 
the uprights may also be employed 





Fig. 253. Form for Column Base at Wesleyan Hall, Westminster 



Fig. 254. Centering for 
Floor at Wesleyan 
Hall, Westminster 

for the same purpose. 
The cross bearers under 
the beam bottom support 
2-in. by 5-in. joists, which 
in turn support the 2-in. 
by 4-in. joists carrying 
the floor slab boards. 
These joists as arranged 
are not the full length: 
between the beams, and 
wedges are inserted be- 
tween their ends and the 
beam boxes to keep the 
latter in position. The 
boarding carrying the 
slab runs parallel to the 
beams, and can be used 
in fairly long lengths, 
thus saving waste in 
cutting. The joists being 
supported direct from 
the uprights, the parts 



FORMS AND CENTERINGS 



of the beams, floor, and columns can be 
easily struck independently of each other. 
Various Column Forms. The form for 



of cement. Figs. 250 to 254 are photo- 
graphic views showing the false-work to 
beams and columns. 




Fig. 255. Centering Resting on Flanges 
of Steel Joists 



a twelve-sided column, illustrated by Fig. 247> 
is made of wooden staves dowelled together 
and hooped with adjustable iron straps. 
The form for a fluted column, shown in 
Fig. 248, consists of narrow laggings dowelled 
together and hooped round with adjustable 




Fig. 256. Centering Suspended from 
Flanges of Steel Joists 

iron bands. The flutes are formed of 
plaster-of-paris attached to the inside by 
screws which are inserted from 
the outside ; in removing them, 
the screws are drawn and the 
plaster flutes remain as a pro- 
tection after striking until the 
building is completed. 

In the new Wesleyan Hall at 
Westminster circular columns 
with entasis are used. The 
forms (Fig. 249) were made in 
sections 4 ft. high, the narrow 
vertical laggings being easily bent to the 
required curve and secured to horizontal 



2 PLANK, 2 8 CENTRES 



Fig. 258. Centering for Arch Ceiling 
between Joist Flanges 

CENTERINGS FOR CONCRETE FLOORS 
CARRIED BY STEEL JOISTS 

The simpler varieties of centering for 
steel joists will now be briefly dealt with. 
Fig. .255 shows a concrete slab lying on 
the top flanges of rolled steel joists ; 
timber bearers rest on the bottom flanges 
and support the lagging. Fig. 256 shows a 
similar centering supported on the bottom 
flanges by means of , hook hangers or sus- 
pended by sb'ngs from the top. It may be 
arched as shown by dotted lines. Fig. 257 
shows a design by W. F. Kearns for a rein- 
forced concrete floor with steel main beams. 
The defect of these three floors is that, even 




- 2-0 CENTRES 
Figs. 259 and 260. Floor Centering supported by Hangers 




2*8 WOOD W4SHERS 
ffe BOLTS HAMD NUT 

Fig. 257. Centering for Concrete Floor 
having Steel Main Beams 

pieces set out with their outer edges 
plumb, while their inner edges coincided 
with the diameter of the column ; these 
horizontal pieces were fixed at intervals of 
2 ft. The method gave such good results 
that the columns could be finished with in. 



although the centering is kept low, the 
bottom flanges of the steel joists are not 
embedded in the concrete, and therefore are 
exposed to the effects of fire ; the defect is 
obviated by having a ceiling on iron hangers, 
metal lathing being embedded in the plaster. 
Fig. 258 shows the centering for an arch 
ceiling between the joist flanges, suspended 
by f-in. bolts. In the method shown by 
Figs. 259 and 260, square-headed hooked 
bars are hung on runners which are placed 
on the upper flanges of the steel joists, 
the bars being placed alternately on each 
side of the joist. 

WALL AND PARTITION FORMS 

The shuttering for walls consists of bat- 
tened widths passed down behind quarter- 
ing driven into the ground and kept apart 
by iron distance pieces at the top, the 



i8z 



REINFORCED CONCRETE 



arrangement for battering the face being on it, to which the uprights, spaced at not 
obtained by sloping the uprights to the more than 5-ft. centres, are fixed and braced 
required -angle. In the case shown by by runners and struts to keep the horizontal 

boards as rigid as possible. The outside 
shuttering is carried up to the full height 



Fig. 262. Section of 
Wall Form 



Fig. 261. Form for 
Wall. Part is Raised 
to Second Position 




Fig. 261 shutters are made of 7-in. by 2-in. 
planks with 4-in. by 2-in. ledgers, and by 
employing sleeve pieces over the f-in. rod 
bolts fitted with heads, nuts, washers, etc., 
the sides of the form are kept the required 
distance apart. In the right-hand part of 
the illustration a section of the wall is shown 
complete, and the shutter has been raised to 
the second position resting on the sleeve 
pieces, as further shown in Fig. 262. In the 
next illustration (Fig. 263) an alternative, 
but inferior, method is shown, the 6-in. by 
l-in. boards having 4-in. by 2-in. continuous 
head and cill pieces. 

In the type of form illustrated in Figs. 
264 and 2(>r> it will be seen that the* floor slab 
having first been laid, cill pieces are placed 



Fig. 263. Panel for Wall Form 



FORMS AND CENTERINGS 



183 



the inner shuttering is made in panels 5 ft. 
wide and 2 ft. 6 in. high, and built up as the 
work proceeds. The size of the uprights is 
governed by their distance apart, the height 
and thickness of wall, and amount of strut- 
ting. This design is not recommended. 
For walls from 6 ft. to 14 ft. high, 2-in. 



is used the studding can be placed farther 
apart, and of course made correspondingly 
heavier in scantling. 

Fig. 266 shows the false-work for a simple 
foundation wall. First, 2-in. by 6-in. hori- 
zontal bearers are laid to 2-in. by 4-in. up- 
rights, the latter having pointed ends which 




5^Z 




2 -G 




K' 6" 






^ OEE - 








snot*-- 


^ 





3TUD 



Figs. 264 and 
265. Plan and 
Cross Section 
of Shored-up 
Form for Wall 




by 7-in. or 3-in. by 6-in. uprights may be 
used if not placed more than 2 ft. 6 in. apart 
or 2 in. by 5 in. if spaced not more than 2 ft. 
apart. The uprights should be stayed by 
one or two rows of horizontal runners 
2 in. by 7 in., or flooring boards propped 
up by 2-in. by 4-in., 2-in. by 5-in., or 3-in. 
by 4-in. struts, whichever scantling is at 
hand. It follows that if 2-in. shuttering 



are driven into the soil ; the bearers are 
kept in position by being secured to stakes 
driven into the ground. The uprights are 
braced and kept the proper distance apart 
by 1-in. by 4-in. battens nailed to their tops, 
vertical packing pieces | in. square being 
placed between the studs to stay the shut- 
tering ; after a section of the concrete has 
been finished, the boards are raised to 



184 



REINFORCED CONCRETE 



continue the wall above. The footing is 
formed by allowing the first layer of concrete 
to flow under the bottom boards. 
Ransome's Wall Form. Fig. 267 shows 




Fig. 266. Form for Simple Foundation Wall 



a form much used in America, and patented 
by E. L. Ransome in 1885. The vertical 
standards are formed of two 1-in. by 6-in. 
boards on edge with a slot between, through 
which pass the bolts (see Fig. 269). By 
undoing the bolt the planks behind the 



joints in each row coincide. Long 6-in. by 
6-in. timbers are next placed vertically at 
the joints, just fitting in the spaces left 
between, the short 6-in. by 6-in. ledges, 
being 3 in. from the ends of the form. Other 
rows are added similarly to suit the height 
of the wall. These long 6-in. by 6-in. 
verticals are kept in position by 6-in. by 
6-in. horizontal wales placed about 2 ft. 6 in. 
apart. 

This set of operations is carried out on 
both sides of the wall ; f-in. diameter rods, 
threaded at both ends and fitted with nuts, 
are passed through the 6-in. by 6-in. wales, 
clear of the 6-in. by 6-in. uprights, through 
the 2-in. planking every 4 ft. ; 2-in. by 4-in. 
distance pieces, the exact width of the 
wall, are placed in the forms quite close 
to the bars, and these distance pieces are 
knocked out as the concrete reaches them. 
After the removal of the forms the pro- 
jecting ends of the iron rods are cut off, and 
covered in with a little cement put over 
the ends. In a particular instance, this 
method of holding the forms proved very 
successful, not the slightest bulging occurring 
although the wall was a thick one. 





Fig. 269. Collar and Set-screw 
at X (Fig. 267) 



standards can be set free 
and the standards raised 
as the wall proceeds. 
The walls are in 4-ft. sec- 
tions. A core box can 
be introduced for hollow 
walls. 

The construction of a 
large panel shutter for 
walls is shown by Fig. 268. 
The panels are made of 2-in. by 8-in. plank 
dressed one side. The panels are erected 
by placing the bottom row in position and 
fixing them by stay lasts. Then another 
row is placed on top so that the vertical 



:'s Wall Form 
/ f*Z PLANKS 


- 


" G"^*xD 

VERTICALS 


n 






' 










7 


- 




^ 


ii (i 


it ii 




11 D 






6*6 


O2- 


' 


6>6 


~C>i * 


rgifi 


3 " 

MARGIN 


(- 














3 


































-1C 












3 O 


1 



Fig. 268. Panel Shutter 

IMITATING MASONRY WALLS 

A method adopted by E. L. Ransome is 
to fasten wood V strips to the wall forms, 
so as to produce an imitation of masonry 
(see Fig. 270). The surface may be left 



FORMS AND CENTERINGS 



185 



plain or rusticated by the application of a 
pick or chisel. 

STAIRCASE FORMS 

The forms on which the stairs at Peek, 
Frean and Co.'s warehouse at Bermondsey 




Fig. 270. Side of Form for Imitating 
Masonry Wall 

were made are shown by Figs. 271 to 273. 
They are in continuous flights, with half- 
space landings. The stringers under the 
soffit of the stair are 2 in. by 6 in., and the 
carriage 2 in. by 4 in., with cross stemming, 



3 in. by 6 in. Horizontal ledgers 2 in. by 
5 in. are fixed to stay the uprights and 
stringer. The soffit of the steps is formed 
by flooring boards being nailed to the 
stringer. The form for the steps themselves 
is constructed of triangular bracket pieces 
nailed to the flooring with a concave moulded 
fillet to form the nosing to the riser (see Fig. 
273). 

In many cases, stairs are made in advance 
and erected similarly to stone hanging steps. 
In that case the stringer is moulded like an 
inclined beam properly reinforced top and 
bottom with rods, and rebated on the lower 
edge to receive the steps. An examination 
of the form for the steps will show at a 
glance that the usual back check in hanging 
steps is got by fixing in triangular fillets. 

METAL FORMS FOR WALLS, ETC. 

A recent system of metal forms for walls, 
columns, and girders is that of the Blaw 
Steel Centering Co., and it is designed to 
combine economically the three processes 



Fig. 272. Stringers 
and Carriage of Stairs 




Fig. 271. Form and Centering for Staircase 



Fig. 273. Form for 
Steps 



as shown in the isometric sketch. The 
stringers are supported at intervals by _2-in. 
by 5-in. uprights ; where the uprights 
require to be more widely spaced, they are 



of form erection, namely, the assembling of 
forms, lining them, and spacing them. The 
panels shown in Fig. 274, which make up 
the wall forms, are based on a standard 



i86 



REINFORCED CONCRETE 



sized surface of 24 in. square. Forms are Corner panels are supplied with returns 12 in. 
furnished in fractional sizes 25 in. by 12 in., on each side and 24= in. high. The panels 




UHER3 
to IO APART 



Fig. 274. Metal Panel Form for Walls 





Fig. 278. Form for Spandrel 
Wall to Bridge 



Fig. 275. Method of 
Fastening Panel 
Flanges together 



Fig. 276. Detail of 
Metal Beam Form 



, 








1 


- 








', 




1 

1 


^ 

$> 

I 


1 

1 


\ 


-~ 






\ 

u, 

', 

\ 


\ 

! 








, 
1 

\ 






Fig. 277. Ransome Form for Cornice 



Fig. 279. Form for Curtain Wall with 
Moulded Cornice 



24 in. by 6 in., 12 in. square and 6 in. square, have angle flanges on all four sides, these 
These sizes enable the contractor to work to being spot welded to sheet steel plates. There 
any dimensions which are a multiple of 6 in. are no rivet marks on the face of the forms. 



FORMS ' AND CENTERINGS 



187 



The plates are joined by special fastenings 
passing through slots in the flanges. The 
flanges are fastened together as shown in 
Fig. 275. Adjustable beam and girder 
moulds are also made, using wooden planks 



' " Rv 

r~^zlh 




wfl!. 



^Bti 



Fig. 280. Two Sections of Form for 
Cornice to Hollow Coping 

as the bottoms. These pieces are so clamped 
together that the sides may be removed and 
used in another part of the work, while the 
bottom board form remains in place securely 
shored until the curing of the concrete is 
complete (see Fig. 276). 




HINGED 
Fig. 281. Form for Ornamental Parapet 

FORMS FOR CORNICES, MOULDINGS, 
ETC. 

Fig. 277 shows Ransome forms applied 
to a cornice. The false-work for a spandrel 
wall to a bridge is shown by Fig. 278. The 
shuttering is 1| in. thick, kept in position 
by 2-in. by 4-in. studs with block pieces. 
The mouldings are made by inserting 
splayed and ovolo or cavette moulded slips 



in the angles of the form. A detail of the 
form for a curtain wall with moulded cornice 
and frieze is illustrated in Fig. 279, while 
Fig. 280 represents forms for the cornice to 
a hollow coping. 



Cf#r- 



Abo 




Figs. 282 and 283. Vertical and Horizontal 
Sections of Form for Battered Retaining 
Wall 

FORMS FOR ORNAMENTAL PARAPET 

Fig. 281 shows the form for an ornamental 
parapet, 2 ft. high, to a bridge or balus- 



i88 



REINFORCED CONCRETE 



trading. Two boards 7 in. wide, 
corresponding to the length of 
the panel, one each for the top 
and bottom, lap over the side 
board, which is 6 in. wide. Two 
4-in. boards are used for one 
side of the rail, and two 2J-in. 
boards to mould the inside edge 
of the rail. 

For the panel centre two 8-in. 
boards joined together are nailed 
to the two 4-in. boards. The two 
7-in. top and bottom boards are 
hinged to this form so as to 
fold back to allow the mould to 
be easily emptied. The two ends 
are of 7-in. timber, hinged to 
close in the ends of the forms. 

FORMS FOR RETAINING 
WALLS 

A form for a battered re- 
taining wall is shown in vertical 
section by Fig. 282, a horizontal 
section on line A B being shown 
by Fig. 283. In this, 4-in. by 
3-in. standards are driven firmly 
into the ground and are provided 
with a cross-head and 4-in. by 
3-in. struts. The 6-in. by^3-in. 
planks are battened together and 
arranged to come alongside the 
4-in. by 3-in. standards. Centre- 
bit holes are made through the 
batten and standard, and f-in. 
bolts, 9 in. long, passed through 
and tightened up. 

Other forms are illustrated by 
three examples, the first of 
which is a retaining wall at 
Bridlington, carried out by 
Ernest Matthews, A.M.I.C.E., 
and clearly illustrated by Figs. 
284 and 285. The height of the 
wall varies from 10 ft. to 13 ft. 
with buttresses at lOJt. centres. 
The cutting is excavated, and 
IJ-in. by 9-in. rough poling 
boards are placed against it with 
two rows of wales in the 
height. These are shored as 
shown with 4-in. by 4-in. struts. 
The foundation slab having been 
put in, the 2-in. by 5-in. up- 
rights, at about 2-ft. centres, 
are erected on the foundation 
slab and spaced apart sufficiently 













V 
s: 








f~/ 


^"UPRIGHTS ^ 


V 




<***STRUT5 
TOCUmMC 

7*2 YVALE5 










7*1"- 


















BUTTRESS 


1 



9" l!4 POLING BOARDS 




Figs. 284 and 285. Plan and End Elevation of 
Form and Centering for Retaining Wall at 
Bridlington 



FORMS AND CENTERINGS 



to allow of the thickness of the wall and 
1 in. of shuttering on each side. The 
uprights are held in position by two rows 
of 1-in. by 7-in. flooring nailed on and 
strutted with 3-in. by 4-in. raking and 
horizontal struts to the cutting as shown, 



with vertical flooring boards cut to the rake 
and nailed to 1^-in. by 5-in. stops, which 
are secured to the uprights. The forms for 
the buttress should be boarded in on top, 
ahead of the concrete laying, with 1-in. by 
7-in. flooring on the slope. The forms are 
wetted both inside and out before 
any concrete is placed in them. 
A 2-in. by 7-in. board, acting as a 
wind strut, is nailed to the top of 
the uprights and the waling at 
the cutting. 

Retaining Wall at Local 
Government Offices. The forms 
for the retaining wall at the 



Fig. 287. Diagram, Plan, 
and Details of Centering 
for Retaining Wall at 
Local Government Offices 




Fig. 286. Part End Elevation and Section of Centering for Retaining 
at Local Government Offices 



Wall 



the shorter ones being 2 in. by 4 in. The 
top and bottom portions of the wall should 
be strutted independently of each other. 
The uprights being securely stayed, the first 
two l|-in. by 8-in. boards are placed on the 
wall side of the uprights. The back is then 
proceeded with similarly. 
The two sides of the buttresses are formed 



Local Government Offices, carried out 
by the Trussed Concrete Steel Co., are 
shown by Figs. 286 and 287. The concrete 
at the basement level is more than 24 ft. 
below the street level, and the excavation 
was done in five sections of roughly 4 ft. 
each. As each section was excavated, 1-in. 
by 7-in. rough vertical sheeting or poling 



IQO 



REINFORCED CONCRETE 



boards were placed against the earth, these 
being secured with 4-in. by 9-in. horizontal 
wales and 2-in. by 11-in. vertical planks 
and raking shores. The shores were 
placed 11 ft. or 12 ft. apart. The top 
and third shores were formed with 
4-in. by 9-in. timbers bolted together, 
the intermediate shores being 7-in. by 
7-in. balks. As will be seen from the 
illustration, the thrust of the earth 
against the retaining wall was resisted 
by trussed and braced buttressed 
division walls with intermediate pilas- 
ters. 

When the earth was properly held up, 
the forms for the wall were begun. 
The wall was 5 in. thick with 9-in. by 
9-in. and 9-in. by 12-in. pilasters. The 
face of the wall is 14 in. from the cutting. 
3-in. by 4-in. uprights were fixed up 
as shown, the boards being 1| in. thick 
and fixed to the uprights a few boards 
at a time as the concreting proceeded. 
The forms for the pilasters were of 
If-in. by 9-in. vertical flooring boards, 
the front being added as the concrete 
proceeded. The column and strut boxes 
were framed on three sides with IJ-in. 
by 9-in. flooring boards, the fourth 
being added in advance of the con- 
crete. The boards were clamped by 
1-in. by 6-in. battens nailed on at 
2-ft. centres. As the wall proceeded, 
the poling boards and wales were taken 
out and the back space filled in with 
concrete. Only the shores were left in 
till the wall was finished, and when they 
were taken out the holes left in the 
wall were filled up. All the boxing 
was of IJ-in. by 9-in. boarding with 
1-in. by 6-in. battens ; the arrises were 
taken off the columns and beams with 
angle fillets. 

Another Retaining Wall. Figs. 
288 and 289 illustrate the method 
adopted in carrying out the deep 
basement in reinforced concrete at the 
premises of the British and Foreign 
Blind Association, Great Portland 
Street, London. The basement has an 
area of 35 ft. by 55 ft., and is 31 ft. 



out in six sections of 5 ft. deep. Imme- 
diately the first section was excavated, 
1| in. upright sheeting was laid to the earth, 








j_ 






J_ 

I 




'i 








E 


toSLic*_ 


I 
I 


sa 








3 

T 


-^- 


<*. 




fc* 






1 
I 






_L 

r 




: 


-.H 


-IMF 




^Hh^ 


3 i^ 


T 


HHI- 


^Hr 






~r 


9-9 




I 

I 

1 

3 
I 


i- 


4" ; 








i 


IOk>*O7i. 


21 




J c 


t - 


I 


I , 


6" 


C . 


L 




T 






pM 




i 


I 


i 


= 


p 


1 



uu ii/. vy uu it., ana is 01 it. p . 
below the level of the pavement. The 
walls on three sides are reinforced 
against a possible 24 ft. head of water. 
The fourth side being an existing brick 
wall is deep enough to render underpinning 
unnecessary. The excavation was carried 



. 288 and 289. Plan and Section of Centering, 
etc., for Retaining Wall in Deep Basement 

waled in the centre with 10 in. plank, and 
strutted right across the basement with 
five balks 10 in. square and 9 ft. apart ; 
folding wedges were used to tighten up 



FORMS AND CENTERINGS 



191 



the struts against the brickwork on the 
fourth side. Three cross struts, 9 in. by 9 in., 
were used to stay the sheeting on the end 
walls, and were continued as stays to the 
large lOJ-in. balks, thus making a rigid 



] L 




Figs. 290 and 291. Elevation and Plan of Form and 
Centering for Silo 



rame. The balks and struts had to be 
placed so as not to hinder the carrying out 
of work on the pillars. The next section 
was proceeded with similarly, until the 
excavation was carried down to the full depth. 
The wall was 14 in. at the bottom, tapering 
in. at the top. Half way up the walls, 



on the outside, large strap beams were intro- 
duced to meet the thrust of the earth and 
water, the thrust being further met ,by 
introducing horizontal reinforced concrete 
strut-beams between the walls and the 
internal pillars, which carry 
heavy weights from the super- 
structure. Horizontally, the 
i wall was strengthened by 
the addition of pilasters. In 
order that the wall should 
be monolithic it was carried 
up in continuous layers. 

The walls and pilasters 
were carried up as described 
for the preceding example, 
the horizontal boarding of 
the walls being secured by 
upright posts, 2 ft. to 3 ft. 
apart, the outer boarding 
being added as the work pro- 
ceeded. The pilaster sides 
were formed by l-in. ver- 
tical flooring, the front of 
horizontal boards being added 
as the work proceeded. As 
previously described, the 
poling boards and walls were 
taken out at the back as 
the work proceeded, and 
only the shores were left in 
till the completion of the 
wall. The boxing of the 
strut beams and columns 
was carried out with 9-in. 
by IJ-in. boarding, and 
clamped with 4-in. by 1-in. 
battens. 

FORMS FOR SILOS 



A typical silo erected at the 
"Hovis" Mill, Vauxhall, is 
shown by Figs. 290 and 291. 
It is difficult properly to re- 
inforce a rectangular corner, 
and for this reason the 
columns at the intersections 
of the silo walls are on the 
angle, as this makes the 
strongest job. 

The moulds for the walls of each bin were 
formed in eight sections, each with planed 
1 J-in. vertical boards 3 ft. 6 in. high, secured 
to top and bottom horizontal ledgers 2 in. 
by 7 in., and a 2-in. by 5-in. middle rail ; 
placed against these were planks rebated 
to receive the ends of the 4-in. by 7-in. 



NOTE FLOORING LAID DIAGONALLY 
OVER. RIBS'. OM THI5 FLOOR. WAS 
MID THE fOfins FOR- COFFERINQ 

FOR/1 roe 




Figs. 292 and 293. Vertical Section and Plan of Centering for Dome 
of Wesleyan Hall, Westminster 



192 



. 



FORMS AND CENTERINGS 



193 



by 9 in. timbers bolted together with 
horizontal cills and cross braces, similar to 
a gantry for a derrick. 

The art of disposing timber to form a 
centre capable of supporting the weight of 
the material placed on it, and ascertaining 




irizontal stays by which the forms were 
kept in position, the proper distance apart, 
and which in turn were cross-strutted with 
4-in. by 6-in. stays. The forms at the angle 
column were kept in position with split 
2-in. by 7-in. vertical and 2-in. by 7-in. 
angle struts placed horizontally, two in the 
height of the form. The forms between 
two adjoining compartments were kept the 
proper distance apart at the head by long 
bolts as shown. The concreting was carried 
up in heights of 2 ft. at a time. 

The forms for the hopper at the bottom 
of the silo consisted of IJ-in. dressed boards 
on 2-in. by 5-in. raking struts carried down 
to the platform and cross braced with 
battens. The upper inside boards were 
laid as the concreting proceeded. 

Forms for circular silos are constructed 
as described later for circular tanks. 

Ffg. 294. Centering for Dome at Annapolis 
New Academy 

the scantling required, demands greater 
knowledge than the ordinary builder usually 
possesses ; but fortunately, in Great Britain, 
at any rate, centres usually err on the side 
of safety. 

The following examples will give a fair 
idea of the nature of dome centering com- 
monly met with in practice. 
Dome of Westminster Hall. At the 
Wesleyan Hall, West- 
minster, is probably the 
finest example of a re- 
inforced concrete dome 
to be found in Great 
Britain, its span being 
109 ft. over the pen- 
dentives. The shell of 
the dome is only 4| in. 
thick, strengthened by 
16 vertical ribs and 7 

I\^ ^/ || || jjj horizontal ribs or rings, 

Fig 296 Fig. 297 three of which latter 

Figs. 295 to 297. -Centering for Octagonal Dome Circular are reinforced to take 

in Section the thrust. For this 

the bottom main rib is 
ORMS FOR DOMES reinforced with an area of 32 in. of steel. 

The centering requisite for erecting domes These cross ribs form an ornamental coffer- 
is formed of numerous uprights carried up ing which is very effective. The centering 
to the springing of the arch from a firm requisite for building this dome was 
footing below, and consists either of scaffold carried on a platform which was built 
poles spaced at 8 ft. to 10 ft. centres, and up from the concrete floor of the hall, 
braced with ledgers and cross braces. In Planks were laid on the floor, and at from 
other cases, the uprights are built of 3 in. 8 ft. to 10 ft. centres uprights consisting o 




JOU2. 






13 



194 



REINFORCED CONCRETE 



three 3-in. by 9-in. timbers bolted together 
were erected, together with horizontal 
ledgers and cross braces. On the top of 
these standards were laid ledgers which 
supported the working platform of heavy 
planks. Between the central uprights in 
the middle of the hall, the lift was erected, 
this carrying material right from the bottom 
to the top of the building, openings being 
left in the floor for this purpose. The plat- 
form was at the level of the main cornice at 
the springing of the dome. The ribs, which 



eight-rib rolled steel joist framework. The 
filling in was on the Columbian system of 
reinforced concrete by cruciform bars sus- 
pended to the ribs by stirrups. An internal 
and external shuttering was used. The 
inner shuttering was nailed to the horizontal 
radiating ribs placed 1 ft. 6 in. to 2 ft. apart, 
resting on triangular brackets fixed to 
3-in. by 7-in. uprights and raking struts, the 
bottom of the latter being secured to planks 
laid on the floor. A portion of the external 
shuttering sufficient for a part of the dome 




Figs. 298 and 299. Reinforcing the Large and Small Half Cupolas of the Poti Cathedral 



consist of two l-in. by 9-in. pieces, bolted 
together, were entirely floored over diagon- 
ally, this flooring supporting the forms for 
the coffering slabs and beams, treated in 
every respect similarly to floors. These ribs 
were supported by struts and uprights con- 
sisting of two 3-in. by 9-in. timbers bolted 
together. Lagging was only erected to 
support the underside of the slab. The 
upper face of the slab concrete was worked 
up from the springing by trowel work. The 
centering is clearly shown in Figs. 292 and 
293. 

Dome of "Morning Post" Building, 
London. This was constructed on an 



was made on the ground, it being'formed of 
horizontal ribs shaped to the contour of 
both plan and section. These ribs were 
braced by 3-in. by 7-in. boards. The dome 
was concreted in sections between the steel 
ribs, the concrete being rammed from the 
top, the same portion of shuttering being 
used for this purpose. When the lower 
portions were concreted the shuttering was 
raised by wire guys. 

A 20-ft. Saucer Dome. The forms 
for a saucer dome 20 ft. in diameter, sup- 
ported on two sides of a barrel vault, which 
was carried out at the chapel of St. Charles 
College, Netting Hill, London, are very 






FORMS AND CENTERINGS 



.teresting. A platform was erected at the 
level of the springing of the arch. From 
this platform, resting on the planks, were 
raised :3-in. by 7-in. uprights wedged up to 
support at the base of the saucer dome a 
circular ring or plate formed of three thick- 
nesses of 1^-in. boarding, which in turn 
supported the bottom ends of 3-in. by 9-in. 
radiating ribs shaped to the contour of the 
dome. On these ribs were nailed close- 
boarded lagging. The arches to the barrel 

,ults were supported on spliced short ribs 



enough for it to harden, then raised 2 ft. and 
clamped in the new position. The moulds 
for the outer form were first put in place, and 
the inner forms were hung from them. This 
was carried up gradually until the men were 
practically working on a floor before the 
crown was reached. The base of the dome, 
which was 69 ft. in diameter, overhung the 
main walls below about 7 ft. all round, the 
diameter of the main building being 83 ft. 
Thus heavy corbelling was necessitated to 
carry the bottom ring of the dome. 




Figs. 300 and 301. Centerings for Belfry and Arches of the Poti Cathedral 



measuring 3 in. by 7 in., supported at the 
joints by 2-in. by 5-in. struts, the ends of 
which rested on a 3-in. by 7-in. tie wedged 
up from the platform. The pendentives 
were filled in with 3-in. by 9-in. shaped 
ribs. 

^ Dome of Annapolis New Academy. 
The dome at the new academy at Annapolis, 
U.S.A., shown by Fig. 294, is a wonderful 
piece of construction. Its peculiarity is 
that no permanent centering was used, the 
mould and the supporting frame being built 
upwards as the work progressed, receiving its 
support from the work already done. The 
forms were kept on the concrete only long 



The cathedral at Poti, Kussia, is described 
in a later chapter, but it is convenient to 
give here a number of photographs showing 
the construction of the dome (see Figs. 298 
to 305). 

Octagonal Dome. Figs. 295 to 297 
show the centering used for an octagonal 
dome, which is circular in section, at a 
mortuary chapel in New York. Fig. 296 
includes a part-plan of the ribs. The 
main section is through the angle ribs, a 
section through the common ribs being 
shown adjoining. The main dimensions 
of the centering are figured in the draw- 
ings. 






FORMS AND CENTERINGS 



197 



FORMS FOR TALL CHIMNEYS 

The most economical material for chimney 
shafts of circular section is reinforced con- 
crete, and the simplest forms for shaft con- 
struction consist of two sets of inner and 
outer moulds, each 3 ft. high (see Fig. 306), 
held together by means of latches which 
can be readily undone to enable the forms to 
be taken away easily. When the concrete is 
filled in to the top of the upper ring the 
bottom set of moulds is released and placed 







Fig. 306. Centering, etc., for Chimney at 
Northfleet 

on the top of the upper moulds, left in 
position and safely held there by the frictional 
resistance of the concrete ; this course is 
repeated section by section till the whole is 
complete. A circular wooden gauge, made 
of two f-in. layers, is placed 6 ft. above 
the level of the top form to hold the rods in 
true alignment, and it is raised as the work 
proceeds. 

As the work is carried up from the inside, 
only a light scaffold is required, built up 
section by section every 4 ft. or 5 ft., the 
framing of each section consisting of four 
uprights to support a square platform of 



stout planks, holed in the centre for hoist- 
ing the materials by bucket. Double-ring 
chimneys are carried up at the rate of one 
form, and single-ring chimneys at the rate 
of two forms a day. 

The Ransome System. The moulds 
and false-work used by the Eansome Com- 
pany in the United States for chimney 
shafts are much more complicated. A 
square scaffold tower is raised in the centre 
of the chimney with four 4-in. by 6-in. 
corner posts, and 1-in. by 6-in. cross braces 
with 2-in. by 10-in. horizontal braces every 
4 ft. or 5 ft., and built up a few feet in 
advance of the construction ; the scaffold 
tower supports a platform of horizontal 
planks. From the tower project beams, 
and from these the inner and outer moulds 
are suspended by means of four vertical 
rods having their upper ends threaded and 
engaging in screw wheel bearings so that the 
lower moulds can be raised or lowered. 
On the top of the form mould on the outside, 
light brackets strutted out also support a 
working platform. To avoid removing the 
cross beams at the head for each set of 
moulds, telescoping scaffolding is built in- 
side. Inside the chimney a staging is sus- 
pended from the tower from which the 
workmen place and tamp the concrete. 
The shell moulds are 12 ft. high. The six- 
sided lower platform is for the workmen 
who finish the outer surface of the concrete. 

FORMS AND CENTERING FOR ARCHES 
AND BRIDGES 

In designing and building forms and 
centering for arches, care must be taken that 
the centering is framed up strongly enough 
to take the weight to come upon it ; that, 
while the material is used economically, the 
work must be capable of being easily taken 
down ; that the footing is strong enough 
to support the verticals without settlement 
when fully weighted ; that the lagging round 
the centering is strong enough to take the 
weight of the arch ring without deflection ; 
that the centering is properly held up in 
position by folding wedges, these allowing 
the centering to be lowered very gradually 
so that the safety of the executed work 
can be tested before finally striking. 

In general, the forms for arches for rein- 
forced concrete are substantially the same 
as for masonry and brickwork, and present 
no difficulty to anyone acquainted with the 
latter. The lagging round the centering 



198 



REINFORCED CONCRETE 



should be dressed to a uniform thickness, 
and to a smooth surface. 

Centerings for Flat Bridges. Concrete 
bridges may be classified under two heads, 
namely, flat bridges and arch bridges. Flat 
bridges are either straight flat slabs or com- 



2 *4 



The false-work is well shown in the photo- 
graph (Fig. 310). 
Centerings for Arch Ring Bridge. 

The centering for a 40-ft. span arch ring 
bridge is shown by Figs. 311 and 312. 
8-in. by 8-in. piles are driven in, sawn 




ELEVATION. 



Fig. 307. Centering for Flat Bridge 



bined slabs and girders, and are adaptable 
for spans up to 40 ft. The centering for a 
bridge of this latter type is shown by Fig. 
307, and, as it bears a strong resemblance 
to beam and floor forms already described, 
it needs no further explanation. 

Centerings for Arched Bridges. 
Arched bridges are constructed with or with- 
out ribs. If the arches are reinforced with 
steel joists or lattice girders, they are known 
as Melan Arches, and the centering is partly 
hung from the reinforcement, as shown in 
Fig. 308. 

Arched Roof Principal at Hammer- 
smith Baths. Fig. 309 shows the forms for 
an arched principal in reinforced concrete at 
Hammersmith Baths, London, one of the 
first of the kind to be carried out in Great 
Britain. In accordance with the usual 
practice, a working platform was carried 
up to the springing. The ribs supporting 
the forms were out of 2-in. by 7-in. stuff, 
in short lengths supported at the junctions 
by raking and vertical struts braced hori- 
zontally with 2-in. by 7-in. ledgers. On 
the ribs were laid 2-in. by 4-in. lagging, on 
which was nailed 1-in. boarding bent to the 
contour and forming the bottom. The sides 
were formed of IJ-in. boarding clamped 
with 1-in. by 3-in. battens, and were sup- 
ported on the 2-in. by 4-in. projecting lag- 
ging. The sides were battened also on top 
to keep the forms the proper distance apart. 



off near the top, transverse runners are 
bolted to the piles, and large folding wedges 
are placed between the tops and the trans- 
verse 10-in. by 10-in. beams, which support 
the weight of the centering above. The 
centering consists of a set of caps or trans- 
verse beams resting on the wedges above the 




Fig. 308. Centering for Melan Arched Bridge 

pile caps, some shaped bearers notched on 
and supported by the upper transonic beams, 
and finally of a closely laid lagging resting 
on the bearers. The last-named are of 
varying size, depending upon the distance 
between the verticals and the weight to be 
carried. For arches having spans up to 




Fig. 309. Elevation of Centering for Arched Principal, Hammersmith Baths 




Fig. 310. View of Centering for Arched Principal, Hammersmith Baths 



199 



200 



REINFORCED CONCRETE 



100 ft. these bearers are from 2 in. to 4 in. 
wide and 12 in. to 14 in. deep, and spaced 
from 1 ft. 6 in. to 3 ft. centre to centre. 
The upper surface of the bearers must be 
curved to fit the curvature of the under 



the work, when the centering was removed, 
was found to be only T 3 F in. 

The main arch, of about 259 ft. span 
with a rise of about 87 ft., is designed for 
fixed ends without hinges. The roadway is 




Figs. 311 and 312. Centering for Arch Ring Bridge 



surface of the arch, and the bearers must be 
braced laterally by 1-in. by 6-in. bridging. 
The lagging consists of f-in. tongued and 
grooved pine or 2-in. spruce with bevelled 
edges. When the bearers are far apart, the 
lagging may have to be as thick as 4 in. 

Teufen Bridge. The bridge at Teufen, 
Switzerland (see the photographs, Figs. 315 



22 ft. 7 in. wide, and is more than 216 ft. 
above the river. The arch voussoir is 
3 ft. 11 in. at the crown and 6 ft. 11 in. at 
the abutments. To carry the centering for 
this arch, twelve verticals six rows deep were 
driven below the river bed and carried up 
above the springing as a platform. Each 
vertical or standard was formed of two 




i :i 



1 




DETAIL AT B. 



Fig. 313. Part Elevation and Detail of Centering for Bridge at Teufen 



and 507) designed by such an undoubted 
authority as Prof. Morsch, presents points 
of great interest. The centering, Fig. 313, 
designed by the professor, is a model to be 
safely followed in similar situations, as the 
permanent deflection at the completion of 



12-in. by 12-in. balks bolted together and 
stayed with ledgers and cross braces, both 
longitudinally and transversely, at every 
29 ft. of height. On top of these standards 
were placed 9-in. by 12-in. ties cleated 
with angle irons on the under-side to the 



FORMS AND CENTERINGS 



2OI 



verticals ; a 12-in. by 4-in. by 40-in. sole-piece 
was bolted to the ties, on which was laid 
two sand boxes, except on the end row 
adjoining the abutments. A similar sole- 
plate rests on the sand boxes, and supports 
the tie beam of the arch centering proper. 
Pieces of channel iron were employed to 



from 4 ft. to 5 ft. wide and the whole width 
of the bridge were left unconcreted ; these 




Fig. 314. 



-Detail of Bridge Centering 
(see A, Fig. 313) 




distribute the pressure of the vertical and 

horizontal timbers on to the sole-plates and 

cross beams of the scaffolding direct on to 

the verticals, and special steel stirrup ties 

held in the feet of the struts, as shown in 

!Fig. 314. The main struts were 10-in. by 

10-in., and the secondary and small struts sections of the ring were strutted apart by 

9 in. by 9 in. and 5 in. by 7 in. respectively, heavy balk timbers, as shown. After the 



Fig. 315. View of Part of False-work for 
Bridge at Teufen 




VERTICALS 



8*8 + 

6*4 



Figs. 316 and 317. Centering for Almandares Bridge, Havana 

The struts supported the shaped stringers ring had taken its proper bearings, the con- 



and laggings, each stringer being composed 
of two 13J-in. by 3j-in. planks. The arch 
ring was concreted in sections, and portions 



creting of the omitted parts of the arch ring 
was completed. 
Arch bridges formed of a series of arched 




Fig. 318. Centering for Meadow Street Bridge, Pittsburg 




STEEL ROLLERS 



'Ll^, ANCHOR/ BOLT3 



Figs. 319 and 320. Centering for Bridge of 233-ft. span 




203 




u 





204 



FORMS AND CENTERINGS 



205 



ribs supporting slabs have the ribs generally 
first erected on such centering as has been 
described for the Teufen Bridge. For com- 
pleting the slabs after the ribs have hardened 



BRACES 2*8 




Fig. 327. Centering for Bridge of 80-ft. span 

somewhat, false-work similar to that de- 
scribed for floors is used. 

Seventeen examples of bridge centering 
culled from the best Continental and 
American practice are shown in outline or 
in photographic view by Figs. 316 to 332, 
the sizes of the main timbers being given as 
far as obtainable. 

SUSPENDED CENTERING FOR BRIDGES 

In the building of bridges over running 
streams great difficulties have been experi- 
enced owing to the difficulty of erecting the 
centering and owing to the serious settle- 
ments of the centering that frequently take 
place. American engineers found that the 
running water washed round about the base 



as at Teufen, and the voussoirs between 
these sections not yet concreted in. The 
voussoirs were raised to their true position 
by screw-jacks, which had to be continually 
adjusted to counteract the sinking 
still going on. This led to the intro- 
duction of flexible suspended center- 
ing, and the particular design about 
to be described was used in Chick- 
ahominy Eiver Bridge, Eichmond, 
Va., U.S.A. Fig. 333 shows cables 
hung over the piers and anchored at 
the abutment of the arches. At- 
tached to these cables are the 




Fig. 328. Centering for Bridge of 110-ft. 
span 

hangers which support the centering shown 
in Figs. 334 and 335. In order to pass 





)0>JZ 



Figs. 329 and 330. Centering for Bridge at Deer Park, U.S.A. 



of the wood piles and destroyed their vertical 
friction, in one case causing a settlement of 
12 in. Disaster would have occurred had 
not the arch been constructed in sections, 



the cables through the piers or abut- 
ments to anchorage, a small pipe or box 
with proper slope is embedded in the abut- 
ments when constructing them. 



206 



REINFORCED CONCRETE 



The arch can be raised or lowered to the 
proper curvature by lengthening or shorten- 
ing the hangers, by means of turnbuckles, or 
by differential pulleys for very heavy loads. 




Fig. 331. Centering for Flat Bridge of 
42-ft. 7-in. span 

It is usual to set up the arch ribs in sections. 
The key spaces are then filled with concrete, 
embedding the reinforcement and binding 
the arch sections together. The arch ribs 
being completed, the cables and centering 
may be taken down and any superstructure 
desired built on the top of the rib. 

Figs. 334 and 335 illustrate a panel of 
suspended centering which is constructed 
of wood firred on top with shaped member. 
The furring member can be changed to suit 
n arch of different radius. The longitudinal 



adopted in Ireland for a foot bridge 172 ft. 
long, and 150 ft. above the level of the sea. 
The ribs were made in four portions on the 
shore, and supported by overhead cables in 
position as before described till the key 
spaces were filled in. The floor was shuttered 
in situ, being supported on the completed 
ribs. 

Figs. 336 to 338 are photographs which 
further illustrate the method of building 
a suspended flexible centering. Near the 
abutments are frames or towers, from the 
top of which hang cables anchored in any 




SPAN 88' 6" 

Fig. 332. Centering for Flat Bridge of 
88-ft. 6-in. span 

suitable way. Wire rope hangers, depending 
from the cables, carry cross members of wood, 
iron or steel, upon which rest longitudinal 




Fig. 334. Elevation of Panel of Suspended Centering 



CABLE 



-CABLE 



CABLE 








i.*"-'*^' ' 'j.'./i ' 
-^CONCRETE RIB : 

v .^-"s',-' \ . . .- 




















J 


f 


fifASMPe -tFET 


1 


^ 






I 




1 







Fig. 333. One of the Three Spans of the 
Chickahominy River Bridge 



Fig. 335. Section of Panel of 
Suspended Centering 



members are 2-in. by 12-in. planks, over- 
lapped at the crossbars and notched to 
receive them. The cross bars are 3-in. gas 
barrel, 4 ft. long. 
A similar style of construction has been 



wood or steel shapes so attached that the 
whole centering is flexible, and any curve 
may be imparted to it merely by varying the 
lengths of the hangers ; for the finer adjust- 
ments, the hangers have turnbuckles. The 




Fig. 336. Gables supporting Concrete Sections, showing Key Spaces to be filled in 




Fig. 337. Arch Ribs before Striking Suspended Centering 



207 



208 



REINFORCED CONCRETE 



end sections preferably rest on the abutments, 
the bearings being hinged. The previously 
moulded voussoir sections of the concrete 
arch ribs, or, instead, the moulds themselves, 



smaller ones are constructed by joining short 
lengths of concrete pipes together and 
sealing the joints, so making one continuous 
pipe. It is not good practice to mould pipes 




Fig. 338. Arch Ribs Formed in Suspended Centering 



may next be laid in place, being supported 
by the side pieces. The approximate lengths 
of the hangers is determined in advance, the 
final adjustments being made by means of 
the turnbuckles or, in the case of very 
heavy work, by means of differential pulleys. 
Targets hang from the cross bars on supports 
of different lengths, and when they all are 
at the same level, it is known that the 
flexible centering has been brought to its 
proper form. An article by Philip Aylett, 
in Concrete (Vol. VII., pp. 24-35), 
should be consulted for further particulars, 



of less than 3 ft. diameter in situ owing to 
the difficulty of devising suitable"^ forms. 
When more than 3 ft. in diameter, the pipes 
can be easily moulded into shapes which 
have more stability and efficiency than 
have those of circular cross section. By 
giving concrete pipes a broad, flat base, 




\ 
RlW 


%'BOLTS 





Fig. 339. Centering for 6-ft. Sewer 

it s author being the designer of the examples 
of flexible centering here illustrated. 

FORMS FOR PIPES AND SEWERS 

The larger concrete sewers moulded in 
place are practically monolithic, while the 



SHEET 
IRON 



Fig. 340. Centering for 8-ft. 6-in. Sewer 

they have a better bearing on the foun- 
dation. 

Fig. 339 shows the centering for a sewer 
6 ft. in diameter, constructed in 8 ft. lengths, 
16 ft. being done at one operation. To 
facilitate striking the forms they are 



FORMS AND CENTERINGS 



209 



constructed in four portions. The ribs are 
2 in. thick, secured to each other by f-in. 
bolts at 32-in. centres, and braced by 2-in. 
by 6-in. bracing secured to the f-in. bolts. 
The top section is secured in position by 



shown. The 12-in. by 1^-in. ribs at 18-in. 
centres are secured by 2-in. by 8-in. braces. 
Fig. 342 shows the form for an arched 
culvert 5 ft. wide. After the soil has been 
prepared for the concrete, the 4-in. by 4-in. 





Fig. 341. Centering for 
Conduit at Jersey City 




Fig. 342. Centering for 5-ft. Arched Culvert 



cills should be set and braced with 
1-in. by 6-in. braces. The inner 
arch form is then wedged up from 
the cills. The ribs are shaped out 
of 1^-in. stuff and placed 3 ft. apart 
centre to centre, this necessitating 
thick lagging (2 in. by 3 in.) ; 1-in. 
stuff would do if the ribs were only 
2 ft. apart. On the outer side, 1-in. 
by 4-in. or 1-in. by 6-in. horizontal 



rn 



Fig. 343. Centering for 
8-ft. Arched Culvert 

12-in. by 6-in. wedges. 
There is f in. clearance 
between the top section 
and the side section for 
dropping the centre. 2-in. 
by 2-in. laggings are fixed 
to the ribs and covered 
with No. 27 gauge sheet 
iron. 

Fig. 340 shows the form for a sewer 
8 ft. 6 in. wide. This pipe was constructed 
in six sections, and made in 7-ft. lengths. 

Fig. 341 shows the centering for the 
conduit for water supply for Jersey City, 
which is constructed in seven sections as 

14 




Fig. 344. Form for 
Small Box Culvert 



boards are laid to 2-in. by 4-in. braces and 
uprights set to the splay of the extrados, 
as shown. 

Fig. 343 shows the form for an 8-ft. arched 
culvert. The 1-in. boards of the wall form 
are secured by 2-in. by 4-in. verticals staked 



2IO 



REINFORCED CONCRETE 



into the ground 'and strutted against dump- 
ing left in ; a '4-in. by 4-in. cill is fixed, 
and on top of this is laid the wedges, at 
least 3 in. deep, to facilitate the removal 
of the arch forms, which are framed of 1 -in. 




Fig. 345. Collapsible Steel Centering for Sewer 

stuff, 2 ft. 3 in. apart. The side forms can 
then be easily removed after the arch forms 
are struck. 

Fig. 344 shows the form for a small box 
culvert 24 in. wide, the top splay giving it 
an arch effect cheaply. The outer form of 



the wall consists of 2 in. by 10 in. boards 
secured to 4 in. by 4 in. posts driven into 
the ground and braced by 1 in. by 3 in. cross 
pieces. The inner forms are of 1 in. boards 
propped up by 2 in. by 4 in. head pieces, 
and cills and posts, which are placed without 
nailing. The form is struck by pushing the 
2 in. by 4 in. head pieces off the side posts. 
Collapsible Steel Centering. A 'great 
deal of sewer work is carried out by the 
Blaw collapsible steel centering, which con- 
sists of flexible steel plates bent cold to the 
proper radius and stiffened with channel 
ribs (as in Fig. 345). The sections are 5 ft. 
long, and held together at the ends by 
staples passing through slots on the male 
end of each section. The centres are held 
in position by two tension rods attached 
by a turnbuckle, these being also used for 
collapsing the centres. The rivets being 
countersunk present a fair face to the 
concrete. The following briefly describes 
the method of their use. A dish is prepared 
in the trench, on which to place the centre. 
The concrete is then placed in position and 




Fig. 346. Conduit at Woolwich, showing Collapsible Steel Centering in Use 



FORMS AND CENTERINGS 211 

jammed ; after it has set, the centres are FORMS FOR TANKS 

ollapsed by means of the turnbuckles, which Form for Square Cistern. Figs. 347 

ase them from the concrete without any and 348 show forms for the erection of a 
arring or hammering whatever. When 



4*2- 




p 


F n 


\-r-t- . 

4*1 




\ 

r^M 


) ' 

9 


J [ 





4*2 




Figs. 347 and 348. Form for Square Tank 



be centres are collapsed they are dropped 
rollers and pulled along ready for the 
ext section. When the centres are very 
irge they are mounted on small wheels run 
n light rails. The centering is adapted to 



small cistern 4 ft. 6 in. by 4 ft. 6 in. by 6 ft. 
high. 1-in. dressed boards are cut to the 
exact inside dimensions, namely 4 ft. 6 in., on 
two sides, and 4 ft. 4 in. on the other two 
sides, and they are held in position by 





Figs. 349 to 352. Form for Circular Tank 



Afferent shapes for all kinds of conduits 
ad for concrete pipe moulds. 
Fig. 346 is a reproduction of a photo- 
j:aph clearly showing the Blaw co lapsible 
i :ntering in use. 



,6*6 



J ROD 



2 *4 LAGGING 

Fig. 351 

2-in. by 4-in. posts strutted across 
with 1-in. horizontal boards at top 
and bottom. Eight 2-in. by 4-in. 
posts at the corners are set on the 
ground, and held in 
position by 2-in. by 4-in. 
inclined strutting nailed 
to planks on the ground 
secured by stakes. The 
inner and outer forms 
are kept the proper dis- 
tance apart by 1-in. 
battens nailed to both 
posts. The posts may 
be much higher than the 
tank in preference to 
cutting the material. 
The outside boards can 
be allowed to project beyond the corners 
and thus save needless cutting. 

Form for Circular Tank. Figs. 349 
to 352 show forms for a circular water tank 
24 ft. in diameter. Six segments of 60 






212 



REINFORCED CONCRETE 



degrees each form the circumference, and 
they are 4 ft. high. There are two bands 
of spliced 2-in. by 8-in. planks at top and 
bottom shaped to the circle, and nailed to 
these at 8-in. centres are 2-in. by 4-in. 
laggings to which are secured No. 22 gauge 
galvanised iron. The yoke, formed of 
6-in. by 6-in. verticals, 2-in. by 6-in. braces 
and adjusted by a turnbuckle, is the means 
by which the outer and inner forms are 
kept the proper distance apart, it also 
enabling the forms to be raised as the work 
proceeds. 

Form for Gasholder Tank. Fig. 353 
represents a section through the walls of 
the gasholder tanks at Dubuque, erected for 
the Key City Gas Company. The bottom is 
about 5 ft. below the outside ground level, 
and the walls rise to a height of 21 ft., being 
18 in. thick at the base, tapering to 12 in. 
at the top. The concrete was first laid over 
the entire bottom 16 in. thick, but increased 
under the walls to a thickness of 2 ft. 6 in. 




round the heads of the posts was also dished 
out in order that the posts could be cut 
down in the striking of the scaffolding, and 
the holes filled up. The whole 264 ft. 
circumference of the tank wall was filled 
up to the level of the top of the form in one 
operation. Forms were used to make a 
mortise and tenon joint between one day's 
work and the following day's. Between the 
outer side of the wall and the earth, up to 
the higher ground level, concrete was filled 
in, well rammed and tamped, and it was 
relied upon as a support to the base of the 
wall. Two 2-in. by 12-in. planks nailed on 
top of temporary posts were laid radiating 
to the tank wall. On top of this two layers 
of 1-in. by 6-in., spliced in short lengths and 
shaped to the radius of the wall, were fixed, 
braced in position with 2-in. by 6-in. up- 
rights and 2-in. by 4-in. inclined braces. 
The scaffolding supporting the wall forms 
on the inside of tank consisted of 4-in. by 
6-in. cills laid on the ends of the post, which 
projected 8 in. above the floor ; from these 
were erected double 2-in. by 4-in. verticals, 
with 4-in. by 6-in. ledgers and 2-in. by 6-in. 
cross bracing. The outside alternate pilasters 
and piers were built up with two sides 
complete, and the third side added as the 
work proceeded. The shuttering to the 
outside wall was similar to that on the 
inside and was fastened in position be- 
tween and under the edges of the pilaster 
forms and lag screwed to the 4-in. posts. 
The edges of the inside shutters were bolted 



4x4, 6*1 PLANKS 



.2*8 




~sr 
1 






\r- 

3 A HOLE 
'/ 2 POO WITH 3" EYE END 

Pit. ^54 




2*12 



Fi. 353 
Figs. 353 and 354. Form and Centering for Gasholder Tank 



As the foundation was uncertain, piles were 
driven 4 ft. 6 in. apart to support the floor. 
The last named was roughly dished at the 
bottom of the wall in order to give a good 
key both to the bottom of the wall and 
to support the scaffolding of the wall forms, 
which were fixed to temporary posts driven 
where shown to carry them. The concrete 



to the sides of the inside 2-in. by 4-in. 
verticals of the scaffolding, and the little 
triangular spaces between the shutters were 
filled up with 2-in. vertical strips of steel 
on a backing strip of wood 1 in. square. 

The forms were raised 3 ft. at one opera- 
tion, by blocks attached overhead to a cap 
piece, and as the forms were 4 ft. high 



FORMS AND CENTERINGS 



213 



there was an overlap of 1 ft. on the finished 
concrete. The forms were cleaned and oiled 



after every operation. Fig. 354 shows the 
inside shutter for the tank. 




Fig. 355. Form for Gasholder Tank at San Sebastian 




Fig. 356. Gasholder Tank at San Sebastian 



A photographic view of 
the forms, etc., for a gas- 
holder tank at San Sebas- 
tian is presented by Fig. 
355, the finished tank 
being shown by Fig. 356. 

Form for Rectangu- 
lar Reservoir. In an 
example of a rectangular 
reservoir having walls 
strengthened by pilasters, 
the slab roof was carried 
on rib beams. In excav- 
ating for the walls, the 
footings were made slightly 
larger and a 2-in. plank on 
edge inserted. After plac- 
ing concrete for the foot- 
ings, horizontal shutter- 
ing of 2-in. board, dressed 
inside and propped up by 
4-in by 4-in. uprights 
placed at 4-ft. centres, was 
erected. For the curved 
corners thin boards were 
bent to the required curve 
and secured to the up- 
rights, the latter being 2 
ft. apart. The posts on 



214 



REINFORCED CONCRETE 



the outside were propped up by raking 
struts and wires passing through the wall 
to the inside, there being braces connected 
to the adjoining columns and walls, and also 
struts extending to a proper bearing. As 
before, the two sides of the pilasters were 



are cheaper, as they require no upkeep. 
Figs. 357 and 358 show the form for square 
posts. A plank is laid as a base dressed 
on the upper side, and on this are set two 
tapering pieces, 2 in. by 6 in. at one end 
tapering to 2 in. by 4 in. at the other. 



5-0 




I 


jgl 




pJA 



Fig. 357. Elevation, Plan, and End View of 
Form for Tapered Square Posts 



m 



Fig. 358. Cross Section 
(enlarged) through Form 
for Tapered Square Posts 




6-lS 1 



1 



Q 



Fig. 359. Plan and Elevation of Multiple 
Form for Tapered Square Posts 

completely formed, and the third side added 
as the work proceeded. The slab ceiling 
and beam forms were carried out as previ- 
ously described for floors, with 2-in. boards 
dressed on one side, both to the beams 
and slab. The beam forms and slab cen- 
tering were erected on 4-in. by 4-in. up- 
rights resting on the finished floor beneath 
and cross braced at intervals. 

FORMS FOR FENCE POSTS 

Reinforced concrete fence posts have been 
used with success, and although their first 
cost is more than wood, in the end they 




Fig. 360. Section of Form for 
Triangular Posts 



These tapering pieces are held in position 
without nails by the side fillets which are 
nailed to the base board. The tops of the 
sides are stayed as shown. Fig. 359 shows 
a multiple form for moulding four fence 
posts at a time, the posts being tapered 
four sides. The divisions separating the 
posts are slipped in between cleats at each 
end, and held in place by wedges against 
blocks nailed to the platform. The illus- 
tration is self-explanatory. Fig. 360 shows 
a mould for triangular posts. The frame is 
hinged so that posts having different angles 
can be made in it. 



Systems Described 



IN this chapter will be found descriptions 
of a number of the best-known modern 
systems of reinforced concrete, based on 
information supplied by various firms in 
the industry. It will thus be understood 
that where special claims are made for this 
or that arrangement, it is simply the in- 
ventor's or owner's point of view that is 
presented. The explanations have been 
kept as concise as possible, since, as most 
readers already know, the firms responsible 
for the various systems issue lengthy and 
well - illustrated descriptions, which may 
generally be had for the asking, and it is 
therefore a simple matter for the reader to 
follow up any special claims or other points 
of interest here briefly stated which may 
invite his further study. 

ARMOURED TUBULAR FLOORING 

A- typical floor on this system consists 
essentially of three parts (see Figs. 361 and 
362), the concrete webs A, coke-breeze tubes 
B, and a concrete top layer c. The webs are 
reinforced with a ribbed corrugated bar as 
shown, and these, together with coke-breeze 
tubes, which are 9 in. long, are delivered 
complete on the site. First the webs are 
placed in the proper position, then the tubes 
are put in between, and the top layer of 




transverse wires, the two being electrically 
welded together at the points of inter- 
section. The wire fabric can be supplied as 
bright steel or galvanised ; either of the 
strands may be of any thickness, from 
No. 4 to No. 12 gauge (Imperial Standard 
Wire Gauge), but No. 5 is the heaviest that 
can be used when both strands are of the 
same size. The longitudinal wires are spaced 
3 in. apart, and the standard spacing of the 
transverse wires is 12 in., 16 in., or 18 in. 
The maximum number of longitudinal wires 
is 24. The standard maximum width is 72 in. 
The greatest width possible is 97 in., and 
the wire fabric is supplied in rolls or sheets, 
a roll of the heavy gauge containing 150 ft., 
and of the lighter gauges 200 ft. 

COIGNET SYSTEM 

Beams and Slabs. The essentials of a 
Coignet beam are shown by Fig. 363, in 
which A indicates the principal bars subject 
to tension, B secondary bars working in com- 
pression, C stirrups to resist shear, D principal 
bars in floor slab, and E secondary bars in 
floor slab. The principle of the beam is the 
introduction into the compressed portion of 
the concrete of the secondary bars over which 




Figs. 361 and 362. Armoured Tubular Floor 



concrete is spread over the whole. The 
ceiling side of the floor may be left as laid, 
or may be finished by plastering. 

CLINTON SYSTEM 

Slabs. This system depends on the use 
of an electrically cross-welded steel wire 
fabric, consisting of a series of parallel 
longitudinal wires spaced at certain dis- 
tances apart, and held at intervals by 



the stirrups or shear members are hooked. 
The stirrups or shear members, which con- 
nect together the concrete working in com- 
pression and the bars working in extension, 
increase the resistance to shearing and also 
the compressive resistance of the concrete. 
Where the stirrups pass round the bars, they 
are fastened by means of annealed wire so 
as not to be disturbed when the concrete 
is applied. The skeleton reinforcement is 



2l6 



REINFORCED CONCRETE 



prepared in advance, and stacked till ready 
for use. As the spacing of the stirrups is 
proportional to the shearing stresses, they 
are closer together near the supports where 
the maximum shearing stresses occur. Much 
importance is attached to the fact that both 
stirrups and the main bars are of round 
section, so that the line of contact between 
them is a mere line. 

Fig. 363 illustrates an early type of beam, 
which has been largely superseded by that 
shown by Fig. 364. 

In a later type of Coignet beam (Fig. 364) 




Fig. 363. Coignet 
Beam and Slab 





Fig. 366. Section 
through Coignet 
Column 



Fig. 367 and 368. 
Base of Coignet 
Column 



the lower bars shown in the previous figure 
are replaced by a group of bars of smaller 
diameter, their ends being bent upwards at 
an angle of 45 degrees and hooked over the 
top bar. The sectional area of the reinforce- 
ment is greatest in the middle portion of the 
beam where the bending moments are higher, 
and the bars are spaced closer together near 
the supports as in the previous example. 
The bars are bound together with annealed 
wire as before. The bars do not lie in con- 
tact one with another, there being a space 
of at least J in. between them into which 
the concrete flows. 



The reinforcing of the floor slab has already 
been shown in Fig. 363. The principal bars 
D are of such diameter and spacing as will 
best resist the tension, the secondary bars 
E merely distributing the efforts more easily 
on the principal ones. At alternate inter- 
sections of the bars, they are bound together 
with annealed wire. Coignet floor slabs 
usually vary in thickness between 3 in. and 
6 in. ; but should the thickness exceed this, 
a double reinforcement is advised, stirrups 
connecting the upper and lower bars at 
intervals. 




Fig. 365. Beam Supporting Floor Slab 
Centering 



Fig. 364. Coignet Beam Reinforcement 
Consisting of Group of Small Bars 




Fig. 369 and 370. Coignet Pipe or 
Conduit 



Coignet beams may be prepared in advance 
and transported and placed in position like 
steel girders. They are made in a horizontal 
wooden mould, the principal bars being left 
protruding for about a foot at each end so 
that they may be fixed on to their supports 
by means of cement grout. Fig. 365 shows 
how the ready-made beams may be used to 
support the centering for the floor slabs. The 
upper ends of the stirrups project so as to 
form a mechanical bond. This method is 
seldom used. 

Columns. A simple type is shown by 
Fig. 366. In this, the vertical bars are 



SYSTEMS DESCRIBED 



217 



bound by horizontal or spiral hoops or ties 
to keep them in their proper position and 
also to resist a bursting tendency. The 
hoops are fastened to the verticals by means 
of annealed wire. The footing of the pillar 
where it is necessary to spread the weight 
takes the form shown in Figs. 367 and 368. 

Walls. As these are to resist lateral 
pressure only, and not intended to support 
any vertical load, this being carried by 
beams, a meshwork of vertical and horizontal 
bars is placed in the centre of the wall to 
facilitate concreting, the intersections being 
bound together with wire. 

Pipes. The reinforcement takes the 
form shown in Figs. 369 and 370, and the 
pipe or sewer is constructed in lengths of 
about 4 ft. or 5 ft., and connected to the 
next section by means of joint and spigot 
as shown ; alternatively, the pipes may be 
concreted in continuous length. 

Piles. Practically, these are pillars 
made in a horizontal mould instead of a 
vertical one. Generally, as in Fig. 170 
(p. 133), they are circular in section, with 
two flat longitudinal surfaces which assist the 
proper guiding of the pile during driving. 

CONSIDERE SYSTEM 

Beams and Slabs. This system is well- 
nigh devoid of fanciful methods of shaping 
and arranging the reinforcement, and is 
claimed to rest entirely on a scientific basis. 
In the case of minor beams, the tensional 
reinforcement takes the form of round bars, 
some of which are horizontal throughout, 
and others are bent up at the ends. Where 
a compression member is required, a spiral 



supports at each end, there being, in addition, 
round bars which cross the other reinforce- 
ments at right angles. In large girders 
and trusses, as employed in bridge con- 
struction, the tension members are rein- 
forced in the manner already described, 
whilst the compression members are rein- 
forced with longitudinal bars and spiral coils. 

Columns. Columns are generally cir- 
cular, octagonal, or square in cross section, 
and the vertical reinforcements are straight 
round bars from 4 in. to 6 in. apart, whilst 
the transverse reinforcements consist of 
round rods in the form of coils wound 
spirally round the vertical reinforcements, 
the ends of the coils being bent inwards. 

Piles. Considere has devoted much 
attention to the construction of reinforced 
concrete piles on scientific principles. His 
general form of pile is octagonal, with 
longitudinal reinforcing rods, generally num- 
bering about eight, inside a continuous spiral 
winding of round steel rods, the pitch of the 
spiral being about 2 in. at the middle and 
diminishing to 1 in. at both head and foot. 
The head is of cylindrical shape and for 
about 4 in. is bound with steel coils closely 
pitched, this reinforcing the head to such 
an extent that it will not fracture under 
the driving shock, even though a cap or 
dolly is not used. Illustrations showing the 
application of the Considere system in prac- 
tice will be found on pp. 358 to 360. 

CORR BAR 

This is an American system of supplying 
beam and column reinforcement already 
made up and suitable for immediately placing 





Fig. 371. Corr Bar Beam "Unit" 



Fig. 372. Types of 
Corr Bars 



coil of round steel is inserted near the top 
surface. Occasionally beams are addition- 
ally reinforced against shear by means of 
thin steel rods, which are lapped round the 
tension and compression bars. In addition 
to the above, Considere sometimes inserts 
an extra reinforcement (a spiral coil of 
round steel laid nearly horizontal) in that 
part of the concrete which is in compression 
near the supports of a continuous beam. 
The slab reinforcements between the beams 
consist of round bars bent up over the 



in the forms. Each made-up reinforcement is 
called a " unit," and a typical construction 
is shown by Fig. 371, this type having been 
designed to give, by means of continuous 
stirrup, an efficient web reinforcement. The 
unit for a round column consists of two or 
more verticals with a continuous hooping of 
cold-drawn wire. The American company 
responsible for this system has introduced 
a variety of special reinforcements, including 
deformed bars (see Fig. 372) and a kind of 
expanded steel known as " Corr Mesh." 



218 



REINFORCED CONCRETE 



DENTILE SYSTEM 

Floors. By means of this system, floors 
are constructed having the advantages of 
terra-cotta tile floors and those of reinforced 



portions fitting together to form a closed 
hollow block. The reinforcement can be 
placed in both directions. In the case of 





Fig. 373. Dentile Floor with Mitre Tiles F & 376 - Diamond Mesh Expanded Metal 



concrete. Briefly, upon the forms are placed 
hollow tile blocks closed on all six sides and 
having projecting flanges, which space them 
at proper intervals. In the channels so left 
the reinforcing bars are placed, the concrete 
being then applied and a floor surface being 
obtained by spreading a layer of concrete 




Fig. 374. Dentile Floor with Bridge Tiles 

right over the tiles. Essentially, then, the 
floor consists of hollow tiles with ribs of 
reinforced concrete between them united at 
the top by a thin slab of plain concrete. On 
the ceiling side there is a continuous sur- 
face of tile. Clear spans up to 32 ft. have 
been built and tested, while for light loads, 



Fig. 375. Dentile Floor with "L" Tiles 

and where surrounding conditions are favour- 
able, the spans may be as much as 40 ft. 
Three shapes of tile are used : mitre tiles 
(Fig. 373), consisting of four wedge-shaped 



the bridge tile, shown by Fig. 374, the 
reinforcement is in one direction only. With 
L tiles, (Fig. 375), the reinforcement can go 
both ways. Small flat filling pieces are used 
to put in the square corners between the 
tiles. In all cases the under-sides of the tiles 
are roughened so as to take the ceiling 
plaster. 

EXPANDED METAL SYSTEM 

Expanded metal consists of rolled steel 
of various thicknesses, cut and expanded 
by machinery into meshes of various shapes, 
the material being obtainable in a number 
of different strengths or weights. Diamond 
' mesh is shown by Fig. 376. There are thirty 
varieties of this material, the variations being 
in the size of mesh, thiclcness of strands, the 





Fig. 377. Rib Mesh Expanded Metal 

weight per yard super, etc. The rib mesh 
is shown by Fig. 377, this being made in 
five varieties ; however, for concrete work 
the diamond mesh material is chiefly used. 



SYSTEMS DESCRIBED 



What is known as the expanded steel bar 
is a bar from which a series of meshes has 
been expanded (see section, Fig. 378). It 
is claimed to be a complete unit, comprising 
tension and shear reinforcements, and as its 
members are rigidly connected, they cannot 



219 



e by a number of stirrups clipped 
firmly around the tension bars as shown in 
Fig. 380 and bent over at the top so as to be 
anchored into the concrete, and they also 
insert other bars straight for the greater 
part of their length and bent upwards at 



Fig. 378. Section of Expanded 
Steel Bar 



"*~~ Expanded Metal Lathir.g 



esh Expanded Steel" 



get out of place during the con- 
creting. When expanded metal is 
used in connection with these _E 
bars, the ends of the sheets may 
be wired to or bent over and 
lapped on to the web of the bars, 
thus securing continuity of rein- 
forcement. 

Floors. When these bear on 

the walls, it is necessary to Fi & 379. Four Types of "Expanded Metal" Floors 
leave an offsett, corbel, chase, 




*** Expanded Metal Lathing 



Reinforced Concrete Bel 
Expanded Sttel Bars 



Ik mesh Expanded Steel 



Reinforced Concrete Bea 
Reinforcing Rod: 




or over-sail. The method of constructing 
the floor with expanded metal is described 
on page 152, and Fig. 379 presents sec- 
tional drawings of a few typical styles of 
construction. 

Other Applications. Expanded metal 
has been found applicable to a wide range 
of work which it is quite unnecessary to 
describe in detail. 

HENNEBIQUE SYSTEM 

Beams. Scientific designing rather than 
the use of steel of special shape or high- 
carbon content is the essential of this system. 
Thin bars or rods and stirrups of steel are 
inserted in just those places where the stress 
diagram of the structure shows that the 
resistance of the concrete requires to be 
supplemented. The experts responsible for 
the system believe that if it were practicable 
to apply the teachings of theory to their full 
extent, the steel reinforcement employed 
would be in the form of fine wires receiving 
and transmitting stress throughout the con- 
crete. They therefore endeavour to recon- 
cile theory and practice by using moderately 
small bars as the main reinforcement, and 
small rods, wire, or steel strips as the con- 
necting or auxiliary reinforcement, and all 
these are in forms procurable in the open 
market. They believe that a beam rein- 
forced simply by straight tension bars near 
the lower face would not be satisfactory in 
practice, and therefore they supplement 
those bars in a beam of the most simple 



each end, part of such a bar being shown 
by Fig. 381. Diagramatically, a simple 
beam of this type is shown in Fig. 382, 
which includes a longitudinal section and 
three cross sections. The bars near the 
lower surface adequately reinforce the 
middle portion of the beam, and the inclined 
bars and the vertical stirrups supply the 
additional reinforcement necessary at the 
ends of the beam where tension occurs at 
diagonal planes. The company emphasises 
their belief that the bars and stirrups rein- 
force the concrete against diagonal tension 
rather than against vertical shear, and say 
that what, in beam tests, is commonly 
described as a " shearing " failure, is almost 
invariably a diagonal tension failure. The 
bent-up ends of the bars lie across the lines 
of rupture near the supports of the beam, 
and afford in themselves very secure anchor- 
age. The spacing of the vertical stirrups is 
such as to provide for variations of stress from 
point to point. Being vertical, they facili- 
tate the operation of ramming the concrete 
without risk of displacement, and being 
made with a spring clip at the lower end, 
they are automatically held in position on 
the main bars without either wedges or ties. 
The stirrups form an effective web connection 
between the tension and compression por- 
tions of the beam. The company does not 
believe that any stress or vibration developed 
under working conditions can possibly over- 
come the natural adhesion between the con- 
crete and the plain round bars, but the ends 



22O 



REINFORCED CONCRETE 



of the bars are flattened and opened out to 
form secure anchorage, and at least half the 
bars are bent up towards the supports, 
further to ensure perfect security. In beams 
that are continuous over intermittent sup- 
ports, the ends of the bent-up bars are 
carried horizontally across the supports and 
terminate near the points of contraflexure 
(see Figs. 383 and 334). The inclined bars 



dimensions. The double stirrups will be 
noticed. 

Floors. Hennebique floors are simple 
combinations of beams and slabs. The main 
beams receive the load which is trans- 
mitted from the secondary beams, the latter 
receiving the load transmitted from the 
continuous slab connecting all the beams and 
receiving the super load placed upon the 





Fig. 381.- 



Fig. 380. Henne- 
bique Stirrup 
round Tension Bar 



-Hennebique Tension Bars 
and Stirrups 



I T 

B D 



iUr 



3*cCion on A-O. Section on CD Section on C r 

Fig. 382. Simple Hennebique Beam 




385. Beam Reinforcement with Com- 
pression Bar and Double Stirrups 





Fig. 387 and 388. 
Base of Henne- 
bique Column 



Fig. 389. 
Sheet Piles 



z- ai 



V 



Fig. 390. 

Hollow 

Diaphragm 

Pile 




Figs. 383 and 384. Hennebique Beams Continuous over 
Intermittent Supports 



Fig. 386. Hennebique 
Column 



in each span, and the horizontal ends of 
other inclined bars projecting from the 
adjoining spans, then resist the tensile 
stress occurring in the upper part of con- 
tinuous beams between the supports and 
the points of contraflexure. 

Beams constructed as above described are 
the most economical types, but it is some- 
times advantageous to employ compression 
bars near the upper surface as shown in 
Fig. 385, which type of beam has advant- 
ages when a very heavy load has to be 
carried by a beam of comparatively small 



floor. In a typical instance the main beams 
have parallel reinforcing bars, and the 
secondary beams have single reinforcement. 
In the usual type of Hennebique floor 
half the slab panel on each side of each 
secondary and main beam constitutes a 
compression flange, and, with the beam, 
forms a beam of T-section. The longitudinal 
bars and stirrups of the main beams pass 
into the superimposed slab, and where there 
is compression reinforcement, this is entirely 
within the slab. The Hennebique hollow- 
tube flooring is mentioned on p. 332. 



SYSTEMS DESCRIBED 



221 



Columns. A typical Hennebique column 
is shown by Fig. 386, the vertical reinforce- 
ment consisting of plain round bars, and the 
auxiliary reinforcement being formed of 
links of T V m - steel wire applied in sets of 
four, one link passing round the two verticals 
on one side. The links brace the longitu- 
dinal bars and support the concrete later- 
ally. This method was introduced many 
years ago, and has given satisfaction in 
thousands of cases. 

Figs. 387 and 388 show a column base, the 
horizontal steel reinforcement distributing 
the force transmitted by the vertical bars. 
The lower portion of the concrete is rein- 
forced by a double system of bars laid at 
right angles with one another so as to pro- 
vide for the tensile stresses caused by the 
bending moment developed by the central 
load and the vertical reaction of the ground. 
Kesistance to compression is provided 
entirely by the upper portion of the con- 
crete, and diagonal tension is taken by the 
vertical stirrups applied as in beams. A 
column base of this description virtually 
represents cantilever construction. 

Piles. These can be built of any approved 
shape, that shown on p. 133 being typical. 
The longitudinal bars are bent inwards at the 
toe, but do not bear upon the steel driving 
point, this being anchored into the concrete 
by four straps bent over at the upper end, 
as shown on p. 133. The transverse ties, 
resembling those in the column already 
described, are spaced more closely near the 
top and bottom of the pile than in the 
middle. 

Sheet piles for retaining walls and coffer- 
dams have a head of reduced section (see 
Fig. 389), and on two sides of the pile there 
is a semicircular groove, the hole formed 
by the grooves in contiguous piles being 
filled with cement grout to prevent the per- 
colation of water. Thus a row of sheet 
piles forms a watertight wall. The hollow 
diaphragm pile (see Fig. 390) is constructed 
on the principle that the resistance to 
driving is largely, if not entirely, due to 
friction between the external surface of 
the pile and the earth, and that a given 
volume of reinforced concrete employed in 
the form of a hollow cylinder is more effective 
than if applied as a solid cylinder. The 
reinforcement is practically the same as that 
of the solid pile, but, in addition, diaphragms 
hold in place a consecutive series of tubes 
each about 4 ft. long, the object of the 



tubes being to form the hollow core of the 
finished pile. 

INDENTED BAR SYSTEM 

This system depends entirely on the use 
of a deformed bar, either square or round 
in section (see Figs. 391 and 392). The 
square bar varies from J in. to 2 in. square, 
and from -24 Ib. to 13-6 Ib. per foot run in 
weight, whereas the round bar varies from 
fin. to 1| in. in diameter, the weight 
per foot run at these sizes being -38 Ib. 
and 6'06 Ib. respectively. Strictly speak- 
ing, the bar itself is the " system," the 
company responsible for it having no 
special -method of arranging the rein- 
forcement, but preparing their designs 
in accordance with approved practice, and 
supplying the indented bars to the building 
contractor to be used in accordance with the 
designs prepared by the Indented Bar Coin- 



Fig. 391. Square Section Indented Bar 



Fig. 392. Round Section Indented Bar 

pany or by independent engineers. They 
contend that their bar obtains a more 
reliable grip on the concrete than does any 
smooth bar. The adhesion between smooth 
bars and concrete depends, they state, upon 
the cement particles entering into the micro- 
scopical irregularities in the surface of the 
steel. When a steel bar is subject to a 
tension of 12,000 Ib., or say 5 tons, per 
square inch, it extends by -004 in. per inch of 
length, this causing corresponding reduction 
in the area of cross section, which, although 
extremely small, is sufficient to affect the 
adhesion. They instance a number of tests, 
the results of which go to show that indented 
square bars sustain 1-9 times the stress 
required to pull out smooth squares, and 
indented round bars 3'7 times the stress 
required to pull out smooth round bars. 
Further, it is stated that owing to the fact 
that at a stress of 3 tons per square inch the 
extension in steel is greater than that to 
which concrete can be subjected without 
cracking, it appears to be undesirable when 
smooth bars are used to allow the tension 
in the steel to exceed that amount. With 



222 



REINFORCED CONCRETE 



indented bars, however, the continuous and 
positive bond provided makes it permissible 
for the stress in the steel to be limited only 
by the strength of the material used, this 
enabling a steel of high tensile strength to 
be employed. The draft regulations of the 
London County Council propose that there 
should be a mechanical bond between the 
steel and concrete in cases where steel used 
has a greater strength than 72,000 Ib. per 
square inch. 

When the company's ordinary specifica- 
tion is adopted, the bars are of rolled medium 
steel with an elastic limit of 50,000 Ib. per 
square inch, with an extension of not less than 
15 per cent, in a length of 8 in. ; the breaking 
strength is 90,000 Ib., roughly 40 tons per 
square inch. By the use of such strong 
material there is claimed to be a consider- 
able saving in the weight of steel required 
for a given strength, the factor of safety 
can be increased without extra cost, and the 
omission of bending, splitting, cranking, and 
other devices commonly used for anchoring 
smooth bars is a source of economy. 

JOHNSON'S LATTICE SYSTEM 

This depends on the use of a steel wire 
mesh or lattice (see Fig. 393), manufactured 
in sheets or panels, and in rolls 6 ft. wide to 
about 120 ft., or the rolls can be made longer 
than 120 ft. and any width other than 6 ft., 
and the spacing can be varied to suit 
individual requirements.^ It is unnecessary 



Fig. 393. Johnson's Steel Wire Lattice 

to give full details of the various methods 
in which this reinforcement is applied. It 
is sufficient to say that it is applicable to 
very many purposes, from foundation rafts, 
to floor slabs, walls, etc. Figs. 394 to 397 
show four different methods of supporting 
mesh-reinforced floors. 

KAHN SYSTEM 

Beams. The experts responsible for this 
system emphasise the fact that a properly 
constructed reinforced concrete beam is in 



reality a trussed beam, and they state that 
stirrups can only transfer stress to the main 
tensional member when the two are defi- 
nitely and rigidly connected together, the 
horizontal reinforcement taking not only the 
stress caused from adhesion of the concrete 
to it, but also the summation of the hori- 
zontal components of the strain in each 
of the diagonals. The principles of truss 
action occur (or grow) out of this. The 





































I r ^ v _ _ T ^ 







Zpifc&ttiiuZi 


















































ry~ 














































zrU.' ''-' - -" ''-' d 






3T 














































Figs. 394 to 397. Four Methods of Supporting 
Mesh-reinforced Floors 

company does not believe that the concrete 
surrounding the bars will prevent the loose 
stirrups from slipping, and they therefore 
attach the shear members rigidly to the 
horizontal reinforcement. In the Kahn 
system a bar with two projecting wings or 
stirrups is used, the stirrups being bent up 
at about 45 degrees, so as to cross the planes 
of rupture at nearly right angles (see Fig. 
398). At the centre span, where the hori- 
zontal tension is greatest, the web is left 
intact on the bar, and there serves as an 
additional reinforcement (see Fig. 399). The 
stirrups are part of the original metal of 
the bars, being merely sheared and bent, 
whereas in the original Kahn bar, holes were 
punched in it and the shear members 
fastened in these holes. The Kahn trussed 
bar, it will be understood, is sent to the 
site complete, ready to be incorporated in 
the concrete as a single unit. 

The bar undoubtedly possesses great 
advantages in lintel construction. The prin- 
ciple is stated as follows : A flat arch acts 



exactly upon the same principle as a seg- 
mental arch, and therefore' -if a section of 
the wall above an opening can be converted 
into such a flat arch, 'and a tie member suit- 




Fig. 398. View of Kahn Bar 

-4- 



Fig. 399. Section and Elevation of Kahn 
Trussed Bar 



ably placed so as to receive its thrust, then 
that section of the wall has become in reality 
an arch. This, in short, is the purpose of 
the Kahn lintel, wherein the Kahn trussed 
bar is used as a tie member, the diagonals 
of the bar taking up the thrusts of the arch. 
The lintels can be built for spans as great 
as 30 ft. or 40 ft. Tigs. 400 and 401 show 
the application of the Kahn bar in lintel 
construction. 

Floors. The floor slabs, reinforced with 
top and bottom horizontal bars, the lower 




Fig. 402. Keedon Beam Reinforcement 




Fig. 403. Keedon Column Reinforcement 

of which is a bar of the type already des- 
cribed, form, with a trussed beam, a beam 
of T-section. 

Columns. The Kahn bar is also applic- 
able to column construction. The prongs 
projecting from the bars (the bars are here 



223 

used as vertical members) reach diagonally 
across the column and tie in the main bars 
at intervals of 6 in. to 12 in. Some Kahn 
pillars have helical hooping. 




Figs. 400 and 401. Lintel Reinforced with 
Kahn Bar 

"Hy-rib" is a sheet-mesh reinforcement 
and lathing introduced by the Kahn 
Company. 

KEEDON SYSTEM 

The peculiarity of this system is the use 
of stirrups, hoopings, etc., which are looped 
to pass over the main bars, and are rigidly 
held to those bars by means of wedges or 
keys. Beam reinforcement on this principle 
is shown by Fig. 402, and column reinforce- 
ment by Fig. 403. The rigid shear members 




Fig. 404. 



-Column and Beam Reinforcements 
Keedon System 



or stirrups resist the diagonal tension. Thus, 
the stirrups and hoopings form in effect a 
series of rigid projections, but the bars, of 
course, retain their original cross section, 
and are not deformed in any way. All the 
stirrups, hoopings, and keys are inter- 



224 



REINFORCED CONCRETE 



changeable, and the work of assembling them 
is quickly done. The main reinforcements 
are ordinary merchantable steel bars. Fig. 
401 shows the Keedon system applied to a 
column and beams. 




Fig. 405. Lock -woven Mesh 

Fig. 406. Lock -woven Mesh Floor 

LOCK-WOVEN MESH SYSTEM 

This depends on the use of a special re- 
inforcement, consisting of steel wires woven 
together at right angles and secured at the 
intersections by means of machine-made 




Fig. 408. Floor Slab Supported by Four 
Columns, Mushroom System 

knots, which are of three kinds, as shown in 
the composite diagram, Fig. 405. The 
lengthwise wires, known as carrying wires, 
are heavier than the transverse or distri- 
buting wires, since they take the tensional 
stress, whereas the other wires do little more 



than keep the fabric rigid in the course of 
laying and distributing any accidental loads 
and temperature stresses. The fabric is 
sent out in rolls or in the form of sheets 
cut to size and shape. The firm responsible 




Lock Woven Mesh 





Fig. 407. Fireproof Construction 
with Lock-woven Mesh 

for the system emphasises the advantage of 
using the close-fibre steel, which is drawn 
out under considerable pressure, rather than 
a material in which the fibres have been 
severed by slotting. The method of placing 




Fig. 409. Head of Column, Mushroom 
System 

in position reinforcements of this type is 
explained on p. 152 in the chapter "The 
Erection of a Reinforced Concrete Building." 
Fig 406 shows a type of floor, with alterna- 
tive methods of constructing the beams. The 
reinforcement is carried over the steel joists 



SYSTEMS DESCRIBED 



225 



and sags in the centre of the floor, so that 
it comes close to the under-surface of the 
latter. It is obvious that Lock-woven Mesh 
is applicable to a great variety of reinforced 
concrete constructions. Fig. 407 shows 
methods of fireproofing floor girders and 
columns with reinforced concrete casings. 

MUSHROOM SYSTEM 

This is an American system, in which the 
bar reinforcement to the columns are splayed 
out radially top and bottom so as to be 
thoroughly bonded into the floors. As 
shown in Fig. 408, there are four belts of 
rods crossing the slab from column to 
column, and splayed out over the supple- 



the peculiar formation of the rods around 
the column head, and is claimed to simplify 
centering. It concentrates the maximum 
amount of reinforcement around and over 
the support where the shear is the greatest, 
and eliminates beams and ribs, giving a flat 
ceiling which allows of freer illumination 
from the windows and more convenient 
placing of shafting in the case of factory 
equipment. The system has been built 
and tested for nearly all spans of from 14 ft. 
to 30 ft., and larger spans can readily be 
made. Fig. 409 shows the head of a column, 
and is an example taken from actual prac- 
tice. The columns are spaced 20 ft. 6 in. 
centre to centre each way, and the floors 





big. 414 



Fig. 411. Paragon 
Column, Beam, 
and Floor Slab 



Fig. 410. Paragon 
Stirrups 



mentary cantilever reinforcement at the 
top of the column (this description is based 
on one by the inventor, C. A. P. Turner, in 
his " Concrete Steel Construction," Part I.). 
It will be noted from the diagram that in 
certain areas the rods are only one layer or 
belt in thickness, and a practical test of the 
construction up to the yield point of the 
steel after the concrete has thoroughly set 
shows the development of cracks, due to the 
stretch of the steel, approximately along the 
dotted lines shown in the diagram, these 
lines therefore showing the planes of greatest 
weakness, and between them, in the centre 
of the slab, there being an approximately cir- 
cular flat plate. The inventor demonstrates 
that these lines are nearly or approximately 
the points of maximum moment in the slab. 
The Mushroom system is so called from 

15 



Fig. 412. Paragon Figs. 413 and 414. 
Column Hoopings Paragon Helical 
Column Wrappings 

are designed to carry 150 Ib. per square foot 
live loads. Each column is reinforced with 
eight 1-in. round steel bars, which project 
4 ft. at the top, where, however, they are 
bent at right angles over a f-in. by 2-in. 
band placed just above the bottom of the 
floor slab. The rods flare radially into the 
slab from this band, and extend outwards 
to a distance of 3 ft. 9 in., while two circles 
of steel rods, 8 ft. 4 in. and 4 ft. 6 in. in 
diameter respectively, rest on the radial 
bars and are wired to them. The reinforcing 
bars in the floors extend well over each 
column head, and run parallel and diagonal 
to the lines of the columns. 

PARAGON SYSTEM 

Special forms of stirrups, hoopings, and 
wrappings are the peculiarities of this 



226 



REINFORCED CONCRETE 



system. Dealing first with the stirrup for 
use in beams, heavy floors, and similar 
structures subject to flexure or bending 
loads, the stirrups are relied upon to resist 
the web or shearing stresses ; they lock 
tightly to the main bars, and are claimed 
to give a perfect mechanical bond dis- 
tributing the stresses along the full length 
of the beam or slab to ensure that the con- 
crete and steel act together in taking the 
load. They can be made to lie at any angle 
to make the most suitable form of truss and 
of any required length. They are so shaped 



of hoopings are shown in Fig. 412, and of 
these the first and last may be referred to. 
The first is for solid work, and has four bars 
which pass through the loops shown. The 
arms A extend across the area enclosed by 
the hoop and diverge, their ends being 
turned and bent away from each other. The 
last has the arms curved to form a central 
ring for use where a central column or pipe 
is to be enclosed. 

The helical wrappings (Figs. 413 and 414) 
are claimed to overcome objections to which 
a continuous wrapping is liable namely, 






Fig. 415. Piketty Beam 



Figs. 418 and 419. Cross Sections of Piketty 
Beam with Four Rows of Bars 




Figs. 416 and 417. Piketty Beams with Two and Three 
Tension Bars 







Figs. 420 and 421. Square Piketty Column Figs. 422 and 423. Round Piketty Column 



as to give a minimum metal-to-metal con- 
tact, and they are available in a variety of 
shapes, in which Fig. 410 shows the prin- 
cipal five. Fig. 411 is a comprehensive 
diagram which includes a beam in which 
the stirrups are employed. 

Hoopings are used in columns, piles, etc., 
and are claimed to reinforce the core in the 
planes between the hoops against the bulging 
action, for which purpose they have inturned 
arms, or their arms are twisted together to 
form spiral binds which lie through the 
centre of the core. Each hoop is placed so 
that its arms lie in a different direction from 
those of the adjoining hoops. Five styles 



that the concreting cannot be seen until it 
reaches the top of the column form ; that 
concrete has to be dropped in from the top, 
giving the larger particles a chance of leaving 
the finer ones, and so forming a porous 
space ; and, thirdly, the wire wrapping may 
be injured in the course of tamping, and so 
cause the whole length to be materially 
weakened. The Paragon helical wrapping 
has been sectionised, the ends of the sections 
being made to reinforce the core in the form 
of bonds similar to the hoopings already 
described. A wrapping is threaded by means 
of its loop over the end of a bar, fixed in 
position, and the concrete placed around it, 






SYSTEMS DESCRIBED 



227 



the tamping being done from the open side 
of the form and consequently providing no 
excuse for injuring the wrapping. The 
figure illustrates the reinforcement for a 
circular column, but square columns also 
can be made by this method. 

PIKETTY SYSTEM 

Beams and Slabs. Paul P. Piketty, 
the inventor of this system, considers that 
double reinforcement is absolutely indis- 
pensable, for reasons which are pursued at 
length in the " Handbook " describing the 
system, among them being fastening of 
stirrups increase of compressive resistance, 
and resistance to secondary tensile stresses 
being the result of permanent deformations 
in concrete. Fig. 415 shows two views of 
the reinforcement of an ordinary beam. 
There are two series of bars, namely, 
straight bars, and bars bent up at a third 
of their length. To resist the shearing 
stresses, there are stirrups connecting the 
lower with the upper reinforcing bar and 
inclined as shown ; their ends are bent over 
as shown in the cross section, and all bars are 
fish-tailed to resist any tendency to longi- 
tudinal sliding. The beam shown by Fig. 
416 is on the same lines, but the lower bar 
is hooked and fish-tailed. There are three 
series of bars in the beam shown by Fig. 417, 
whilst Fig. 418 is the cross section of a beam 
with four rows of bars. Fig. 419 is the cross 
section of a beam with symmetrical reinforce- 
ment. The transverse bars are in tension, 
whilst the horizontal links hold the four 
rows of bars together and increase the com- 
pressive resistance of the beam. Slabs on 
this system have both lower and upper 
reinforcement, the bars of one being generally 
at right angles to the bars of the other. 

Columns. Figs. 420 to 423 show pillars 
or columns reinforced on the Piketty system. 
The square pillar has four vertical reinforce- 
ments, and the special shape links will be 
noted. The round column has six rein- 
forcements, the links being on the same 
principle as in the square column. 

OTHER SYSTEMS 

Bonna Pipes and Conduits. These are 
reinforced with ribbed bars formed into 
longitudinals and spiral coils, the two being 
notched together and tied at the joints 
(see also p. 369). 

Chain Concrete Floors. By means of 
special clips, round steel bars laid parallel 



in the floor slabs are connected together to 
form a continuous sheet. 

Columbian Floors. The reinforcement 
comprises ribbed steel bars in conjunction 
with rolled steel joists, or heavier section 
ribbed steel bars alone. 

Dawnay Floors. The reinforcement 
consists of rolled steel joists at 16-in. in- 
tervals, or square bars at 12-in. intervals, 
laid between other joists. 

Ellis Pipes. Round steel rods reinforce 
sections as short as 2 ft. or 3 ft., there being 
an ogee joint to each section. 

Hodkin-Jones Floors. These are re- 
inforced with corrugated bars placed on 
edge and resting on steel girders through 
the medium of a bent and slotted plate. 

Homan Floors. These are of many 
types. In one, the webs of rolled steel 
joists are pierced to allow of the passage of 
round bars which project into adjoining 
slabs ; and in another, the reinforcement 
takes the form of a T-bar with corrugated 
web. 

Koenen Floors. Slabs are haunched 
at each side near the supporting piers or 
joists, the reinforcement of round bars 
lying flat between the haunches and being 
turned up at the ends so as to be securely 
anchored. 

Lindsay Floors. Slabs may be bounded 
by girders on two opposite sides, and by 
rolled steel joists on the other two. Pairs 
of round bars cross, in the vertical plane 
in the middle of the slab, one bar passing 
over, and the other under, the next 
joist. 

Potter Floors. The tensional reinforce- 
ment consists of reinforced corrugated rods. 
When the span exceeds 12 ft., rolled steel 
joists are introduced. 

Ridley Cammell Floors, Columns, etc. 
The beam reinforcement is a trough of 
dovetail corrugated steel sheeting and angle 
bars, the slab reinforcement consisting of 
corrugated sheets. Concrete is applied to 
both sides of the reinforcement, but center- 
ing, etc., is unnecessary, it is claimed. 
Column reinforcement is a combination of 
the corrugated sheet and bars of various 
sections to form a cage. For walls, also, 
the sheeting is used, studs, if required, being 
provided by bars. 

Somerville Floors. Reinforced flat- 
bottomed hollow blocks with curved tops 
are supported by the flanges of rolled steel 
joists, a top layer of concrete completing 






228 



REINFORCED CONCRETE 



the floor. In another type, the blocks, 
much as before, are supported by reinforced 
concrete beams, which, when the upper 
layer of concrete is in place, become tee- 
beams. 

Wells Beams, Floors, etc. The ten- 
sional reinforcement of a beam consists of 
twin bars connected by a web, the bars being 
placed on edge when several are employed. 
Some of the bars may be bent up at the 
ends, and, in addition, there may be straight 
bars to take part of the compression. The 
shear reinforcement consists of vertical 
stirrups, these being known as " hangers " 
when they connect the upper and lower 
reinforcements, and as " bonders " when 
projecting downwards from the compression 
bars to the neutral axis. Slabs connect 
main and secondary beams in the usual 
way to form floors. Rectangular columns 
of moderate size have a round bar near 
each angle, the transverse ties being round 
links with a hook-and-eye joint. In larger 



columns, the links are connected to the 
vertical reinforcements. 

Wilkinson Floors. Floor slabs have 
round bar reinforcements in both directions, 
with their ends bent over for more secure 
anchorage. 

Williams System. Small beams are rein- 
forced near the lower surface with I-section 
bars which take the tension, while the 
shear is taken by vertical round bars split 
at the ends. Larger beams have I-section 
compression bars, and still heavier ones are 
reinforced with railway metals, and with 
diagonal bars so attached to the reinforce- 
ments that the whole is self-supporting in 
the mould. The slabs connecting floor 
beams contain small rolled joists placed, 
parallel and laced together with hoop steel 
passing above and below the small joists 
alternately. Joists with flat steel bars, 
riveted to them are used in piles constructed 
on the Williams system, the end of the 
joist being shaped to act as a driving point. 



The Architectural and Surface Treat- 
ment of Reinforced Concrete 



ALTHOUGH European and American archi- 
tects are now using reinforced concrete 
extensively, it must be admitted that, as far 
as buildings of any architectural pretension 
are concerned, it is almost entirely used in 
hidden situations, such as foundations, floors, 
beams, columns, etc., or perhaps as a backing 
to the external walling, either brickwork or 
masonry, and its use rarely modifies the 
external expression of the building, except 
in the case perhaps of commercial or indus- 
trial architecture. As an example of rein- 
forced concrete building clothed by masonry, 
attention may be directed to Mr. W. Aubrey 
Thomas's Koyal Liver Building, Liverpool 
(see pages 329 to 333), in which the external 
walls are of granite not more than 14 in. 
tlu'ck. In this building the great difficulties, 
from an architectural point of view, of deal- 
ing with an erection of its height have been 
very successfully overcome. 

The statement is frequently made that 
architects do not like reinforced concrete, 
and to a certain extent this is true. Why ? 
Probably the reason is to be found in the fact 
that concrete has little or no inherent beauty. 
Building stones, slates, bricks, and tiles have 
a beauty of their own, either of colour or of 
texture, or both, irrespective altogether of 
the design of which they form a part, and 
this beauty is generally increased by the 
action of the weather. This is also true of 
timber, with its infinite variation of graining 
and colour. This being so, architects have 
not in the past considered reinforced con- 
crete as likely to become a serious competitor 
with traditional materials for external walls. 

Many who visited the Paris 1900 Exhibi- 
tion probably had reinforced concrete 
brought to their notice for the first time. 
This exhibition had undoubtedly much in- 
fluence on the recognition of reinforced con- 
crete in England and throughout Europe 
generally, and later exhibitions have still 
further advertised its claims. The larger 
class of exhibition buildings are admirably 
adapted to advance the architectural claims 
of the "new" material, as in their design a 



degree of freedom may be permissible that 
would not be allowed in more enduring erec- 
tions. This has been, to some extent, taken 
advantage of, but not so largely as might 
be wished. 

In considering the question of reinforced 
concrete and the influence it is likely to 
have on architectural design in the future, it 
must be admitted that a really satisfactory 
treatment typical of the material has yet 
to be found. This will not be achieved by 
any one man, but will only be brought about 
gradually, in the course of time, as the 
material comes into more general use in 
architecture, each architect adding his small 
contribution towards the solution of the 
problem. In other words, it will be a growth, 
just as all past developments have been. 

The guiding principle of the civil engineer 
is to use as little material as possible, and 
this explains why he has so readily adopted 
reinforced concrete, for that is precisely the 
principle on which it is designed. Whether 
it is an altogether good principle from an 
architectural point of view is another matter, 
since the architect in all monumental work 
uses mass as an element of expression, but 
it must be remembered that during the 
evolution of Gothic architecture the loads 
were carried on comparatively small sup- 
ports, and these were reduced more and 
more as the style progressed, the reason 
being most probably purely aesthetic. A 
similar tendency to reduce the amount of 
material employed to a minimum is seen in 
reinforced concrete design ; but in this case 
it is for frankly economical reasons. Instead 
of continuous walling, the weight of the 
building is carried down to the foundations 
on piers, external and internal, the floors 
resting on internal beams or on wall lintels 
spanning from pier to pier, and the exterior 
skeleton being united by thin panel walls 
which carry only their own weight and are 
used merely to keep out the weather. This 
thinness of the external walling is one of 
the drawbacks of reinforced concrete from 
an architectural standpoint, as, owing to the 



229 




Fig. 424. The Upper Stories of a Reinforced Concrete Warehouse at Cologne. 

Front View 




Fig. 425. End View of Reinforced Concrete Warehouse at Cologne 



230 



ARCHITECTURAL AND SURFACE TREATMENT 



231 



difficulty of obtaining effective shadows, a 
flat treatment seems almost inevitable, it 
being impossible to get any depth to the 
window reveals save by subsidiary piers or 
other similar expedients. Tor this and 
other reasons it does not appear likely that 
the traditional materials need fear any rival 
for some long time to come in the largest 




Fi>. 426 



Fig. 427 



and most important architectural work. It 
seems probable that in work of this class its 
greatest opportunities of affecting design will 
be in the construction of domes, vaulting, 
ceiling lights, roofs, and similar works, for 
which the material is so admirably suited 
that it is hard to conceive any other method 
of building by which such good results could 
be so easily attained. 

Inasmuch as there is a temptation to 
overrate the position of reinforced con- 
crete and the influence it is likely to 
have over architectural design in the 
future, it may be advisable to consider 
its possibilities and limitations as a 
constructional material. It is frequently 
spoken of as though its use enabled 
larger spaces to be covered than was 
ever before possible ; but this is not the 
case, and, although we have now in 
engineering constructions some remark- 
able examples of the distances that can 
be spanned by the scientific use of the 
material, the longest span at present is 
to be found in the Grafton bridge con- 
structed by the city authorities of Auck- 
land, New Zealand. The bridge is 960 ft. 
long and 40 ft. wide, and the main arch 



has a span of 320 ft., although this will 
be exceeded by the Albert Bells bridge in 
Rome. It is practically certain that it will 
always be possible by means of steel to span 
larger openings than would be the case were 
reinforced concrete employed. But steel, 
when used in buildings, must be cased, as, 
even if its successful treatment from an 
architectural point of view 
did not render this necessary, 
its lack of fire-resisting quali- 
ties would make it impera- 
tive. This casing, while pro- 
tecting the steel from fire, is 
often a source of positive 
weakness so far as the action 
of the weather is concerned, 
as it prevents the periodical 
painting of the steel. This 
suggests another great advan- 
tage possessed by reinforced 
concrete, namely, its reduc- 
tion of maintenance expenses 
to an absolute minimum. 
This and the rapidity with 
which reinforced concrete con- 
structions can be erected are 
most important factors when 
comparing the cost of erec- 
tions by the two methods. 

Again, it must be admitted that a rein- 
forced concrete beam is a more truthful, 
architecturally and scientifically defensible 
method of construction than a rolled-steel 
joist cased ; and if the former is used in 
connection with a continuous floor slab the 
beams are all converted into T-beams, which 



Fig. 428 



Figs. 426 to 428. Verti- 
cal and Horizontal Sec- 
tions of Small Hall with 
Arched Ribs 




Fig. 429. Interior View of Hall with Arched 
Ribs 

adds greatly to their strength, the whole 
being in monolithic connection with the build- 
ing generally. With reinforced concrete, as 
with steel, where large spaces have to be 



232 



REINFORCED CONCRETE 



spanned, depth in the beam is indispensable ; 
but if the architectural arrangements of the 
interior permit, the beams may be most 
economically constructed in concrete as 
arched beams, so reinforced that no thrust 
is transmitted to the supporting piers. 

Some good examples of arched beams 
carrying very heavy loads are to be found 




Fii. 430 



'o 



Fig. 431 



in the Koyal Liver building previously men- 
tioned, and in the Wesleyan Central Build- 
ings, Westminster (see pp. 337 to 346). 

With regard to stanchions and columns, 
it will be found that these, generally, when 
constructed in reinforced concrete, will work 
out to about the same or slightly smaller 
sectional area as would have been the case 
had a steel stanchion been designed to carry 
the same load and then cased with some 
fire-resisting material in the usual manner, 
so that, as far as proportion is concerned, 
the position is practically what it was before. 
Yet, here again, the advantages claimed for 
reinforced concrete, when used in the con- 
struction of beams, could with equal force 
be claimed for the material when employed 
in supporting them. In connection with 
the above remark on column proportions, 
the two following examples may prove 
interesting. In the first case, the length of 
the column was 20 ft. and the load to be 
carried 200 tons. By designing in reinforced 
concrete the column worked out to 22 in. 
square. By selecting a steel stanchion and 
allowing 2 in. all round for fire casing the 
result was 18 in. by 17J in. In the second 
case, the length of the column was 12 ft., 



and the load to be carried 40 tons ; the 
reinforced concrete column worked out to 
10 in. square, and the steel stanchion, allow- 
ing 2 in. all round as before, to 12 in. by 
10 in. 

It must not be forgotten that reinforced 
concrete is introducing no such revolutionary 
or very great change in architectural ex- 
pression as that which occurred 
when the arch superseded 
the lintel. It is interesting 
to find Prof. Beresford Pite, in 
the course of his Carpenters' 
Hall lecture on " What is 
Artistic Craftsmanship ? " say- 
ing : " If the Greeks had had 
the use of steel girders or 
ferro-concrete material they 
would have been delighted to 
find that they could span the 
whole of a big front with 'a 
beam such as has been em- 
ployed in the new Post Office 
building in Newgate Street." 
It seems practically certain 
that the Greeks had a know- 
ledge of the arch, and that 
they preferred and deliber- 
ately selected the lintel for 
covering their openings, in spite of the 
former rendering larger openings possible 
but it does not follow that they would not 
have welcomed a stronger beam, although 
their desire to span larger openings was not 
emphatic enough to force them to adopt the 



Fig. 432 



Figs. 430 to 432. 
Vertical and Hori- 
zontal Sections of 
Small Hall with 
Barrel Roof 




Fig. 433. Interior View of Hall with Barrel 
Roof 



arch. Allowing that the physical properties 
of marble were the main factors that fixed 
the general proportions of Greek architecture, 
at any rate as far as the spacing of the 
columns was concerned, it must be admitted 






ARCHITECTURAL AND SURFACE TREATMENT 



233 



that no such definite proportions could be 
expected by using reinforced concrete. A 
marble or masonry beam of a certain sectional 
area is capable of doing a definite amount of 
work ; but in the case of reinforced concrete 
the strength of the beam depends so largely 
on the reinforcement which is hidden, that 
no such definiteness is possible, although 
there is a certain proportion between the 
concrete and the steel in a beam which may 
be said to be economically ideal. 

The following quotation from Garbett's 
able book on architectural design is very 
suggestive : "A new style requires the 
generalised imitation of Nature and of many 




Fig. 434 



Fig. 435 



previous styles ; and a new system requires, 
in addition to this (as Prof. Whewell has 
remarked) the binding of all together by a 
new principle of unity, clearly understood, 
agreed upon and kept constantly in view. 
Constructive statics afford three such prin- 
ciples the depressile, the compressile, and 
the tensile methods the beam, the arch, the 
truss, of which the two former have been 
made the bases of past systems ; the third 
is ours, to be used in the same manner." In 
reinforced concrete, the binding principle of 
the truss is introduced into beams by giving 
them the rods to take the tensile strain, thus 
economising material and rendering larger 
spans possible. 

It is to some of the larger manufacturing 
buildings in which reinforced concrete has 



been employed where the mask walls are 
uncased, or to engineering works, that we 
must turn to find treatments which, although 
guided by economical and strictly construc- 
tional reasons, may be said to be most 
typical of the nature of the material. 
Obviously, buildings that are to be hidden 
by masonry are more interesting from a con- 
structional point of view while in the skeleton 
state than after being clothed. The general 
resemblance between their proportions and 
those of steel framework should be noted, 
but we must avoid letting the material in 
question have too arbitrary a sway, as we 
find that from the earliest times, and during 
the most flourishing 
periods of architec- 
ture, design in one 
material has fre- 
quently been greatly 
influenced by design 
in other and some- 
times very dissimilar 
materials. As an ex- 
ample, we have only 
to take the Greek 
Doric order, with 
its triglyphs, guttse, 
and mutules, which, 
although executed in 
marble, are obviously 
founded on timber 
construction. 

Taking, then, an 
ordinary reinforced 
concrete warehouse, 
the piers taking the 
weights will probably 
be found to show on 

both the inside and outside ; the floors, where 
they come to the outer wall, will be carried 
by wall lintels or beams, which also show ; 
the roof will be flat ; the exterior mask walls, 
or panelling, possibly about 5 in. or 6 in. 
thick, and the window openings fitted with 
steel casements. The principal defect (leav- 
ing out the question of surface finish) is 
generally the lack of proportion ; other 
defects are flatness and want of shadows. 
The last mentioned might be partly remedied 
by keeping the faces of the wall piers, wall 
lintels, and curtain walls flush, or almost 
flush, on the inside, and by making the piers 
of such a shape on plan that they will fit 
their position in the general design. This 
can frequently be done without in any way 
increasing their sectional area. If the main 



Fig. 436 



Figs. 434 to 436. Vertical 

and Horizontal Sections of 

Small Hall with Modified 

Barrel Roof 



234 



REINFORCED CONCRETE 



piers are fairly widely spaced, two subsidiary 
piers may in some cases be with advantage 
introduced between them, and if the inter- 
vening windows occupy the whole width 
between the subsidiary piers, more satis- 
factory reveals can be obtained. The pro- 
jection of the cornice could generally be 




Fig. 437 



rectangular, having small projections at 
each end of the main block. The work has 
been carried out entirely in reinforced con- 
crete subsequently treated with yellow 
roughcast, which, being subdued in tone, 
gives a really pleasing colour effect. For 
the roof rough-dressed local slates of a green 
tint have been em- 
ployed in an attrac- 
tive manner. The 
general massing of the 
building is good, and 
the effect of breaking 
up the roof surfaces is 
excellent. It is in the 
massing of the roof 
that the chief charm 
of the building lies ; 
the bold lines of the 
gables and slate-hung 
dormers are effective, 
and the way in which 
the roof surfaces rise 
one above the other 
gives a charm to the 
design. With regard 
to the method in 
which the slates have 
been used there 



Fig. 439 



Figs. 437 to 439. Ver- 
tical and Horizontal 
Sections of Small Hall 
with Pierced Arched 
Ribs 



Fig. 438 



greatly increased with advantage to the 
building. Another way of obtaining good 
horizontal shadows is by means of bal- 
conies, where the character of the building 
renders them desirable. They can be formed 
by continuing the floor slab through the wall, 
the reinforcing rods being continued and 
turned up in the parapet or balustrade, the 
whole being in monolithic connection. Where 
subsidiary piers are inadmissible, and very 
flat reveals are unavoidable, then probably 
casements with broad frames round, in what- 
ever material they might be constructed, 
would give the best results, just as in a flat 
Georgian treatment we have the broad 
frames of the sashes with a mould round set 
nearly flush with the external face of the 
brickwork. 

There is an interesting example of rein- 
forced concrete construction on the left bank 
of the Khine at Cologne ; it is a warehouse, 
two views of which are shown by Figs. 424 
and 425, p. 220. In plan the building is 



workmen. In the 



is 

much to learn from 
the Germans, who are 
apparently far more 
skilled in the use of 
slates than are English 
whole roof a visitor 



could not discover a single instance of the 




Fig. 440. Interior View of Hall with Pierced 
Arched Roof 

use of lead. All hips, valleys, gutters, etc., 
are worked with the slates themselves. This 



ARCHITECTURAL AND SURFACE TREATMENT 



235 



masterly use of the slates, together with 
the well-designed sprocketing, has the effect 
of softening down the roof lines to a remark- 
able degree. Taken as a whole, the design 




Fig. 441. Interior View of Hall with Arched Ribs and Vertical 

Columns 



is most instructive, and helps to show what 
vast possibilities there are for fine archi- 
tectural treatment of factory buildings, 
warehouses, etc., in reinforced concrete. 

Turning to engineering works, particu- 
larly bridges, some very successful and 
architecturally suggestive designs have been 
executed in reinforced concrete. In bridges 
that are typical of reinforced concrete bridge 
design, there are two arched ribs, or, if the 
bridge is wide, a series of arched ribs, with 
vertical columns resting on them, giving the 
effect of open spandrels, and supporting 
longitudinal and transverse beams, which in 
turn carry the road decking. Some very 
light and graceful effects are obtained by 
working on these lines, and the treatment 
also affords suggestions which should prove 
useful in roof designs. 

In the designs which will now be referred 
to designs that have been prepared with a 
view to their suitability for execution in rein- 
forced concrete the endeavour has been to 
show how some of the suggestions previ- 
ously made might be allowed to have their 
influence ; but, of course, the illustrations 
must be taken as merely showing proposed 
treatments, and not as working drawings. 
Figs. 426 to 429 show a cross section, 
plan, the longitudinal section of two bays, 
and perspective outline through a building 



that would be suitable for a small parish 
hall. The springing being so low, the only 
form of wooden principal that could have 
been used would have been of the hammer- 
beam type, and, the 
span being 40 ft., this 
would have made heavy 
buttresses a necessity. 
The proposal is to in- 
sert reinforced concrete 
arched ribs, carried down 
right on to the founda- 
tions the pier being a 
feature inside as well as 
outside and designed so 
as to exert no outward 
thrust, connected by con- 
tinuous reinforced con- 
crete roof and ceiling 
slabs, the former of 
which come down on to 
reinforced concrete beams 
between the arched ribs 
at eaves level, and all 
being in monolithic con- 
nection. Below the eaves 



the outer walls, which are merely mask walls 
and carry no weight, could be constructed 
as desired according to the character of the 
external design and the funds available. 
No reinforced concrete purlins have been 




Fig. 442. Part Longitudinal Section of Hall 
with Arched Ribs and Vertical Columns 

shown, as the idea was by getting away from 
the effect of timber construction to produce 
something more typical of the new material ; 
but if inserted they would, by making a 



236 



REINFORCED CONCRETE 



thinner roof slab possible, cheapen the cost. 
Again, if the arched ribs are looked upon as 
only taking the place of timber roof trusses, 
the rest of the roof could be finished with 
wooden purlins, rafters, etc., in the ordinary 
way, and would be still cheaper, though not 
nearly so satisfactory. 

By means of sections, plan, and outline 
perspective, Figs. 420 to 433 show another 
small hall, but this time with barrel roof, 
the arched ribs showing below same, spring- 
ing from piers and being connected at eaves 
level by reinforced concrete beam?, and 
having continuous reinforced concrete roof 
slabbing and barrel vault, the external walls 
only keeping out the weather as before. 

The plan and sections in Figs. 434 to 436 
show a variation of a barrel roof, with flat, 
panelled ceilings on each side. In this 
design the whole is intended to be executed 
in reinforced concrete, except the mask walls, 
which are in brick. The arched rib principle 
is abandoned, and the tie-beam adopted, and 
although this represents a type well adapted 
for construction in reinforced concrete, it 
yet does not seem so typical of the material 
as the others, but rather more suggestive 
of timber. 

Figs. 437 to 440, with two sections, plan, 
and perspective sketch, show a rather more 
elaborate roof, with pierced arched ribs con- 




Fig. 443. Cross Section of Hall with Arched Ribs and 
Vertical Columns 



nected by reinforced concrete beams at 
eaves level, moulded to form interior and 
exterior frieze and cornice, with roof and 
ceiling slabs, etc., all as before ; but, in 



addition, the pierced ribs are connected by 
arched purlins, and the piers inside and out 
have caps and bases. 

A type of roof founded on reinforced 
bridge design is shown in perspective, part 
longitudinal section, and cross section, by 
Figs. 441 to 443. There are arched ribs at 
intervals, as in bridges, with vertical columns, 
supporting longitudinal and transverse 
beams, which in turn support the roof slab. 
This corresponds to the bridge decking, save 
that it is on the slope, and is treated just 
as a reinforced concrete floor would be 
treated, with main beams over the arched 
ribs, and secondary beams spanning from 
main beam to main beam. A continuous 
skylight is shown ; the beams supporting it 
are connected by a series of arched ribs with 
open spandrels springing from main arched 
rib to main arched rib. The rest of the 
design is sufficiently explained by the draw- 
ings. This represents a form of roof design 
capable of great development, and perhaps 
more typical of reinforced concrete than 
any of the others. 

Figs. 444 to 448 show a design for an exhi- 
bition hall, which is illustrated by means of a 
longitudinal section showing two bays out of 
three, plan of same, outline perspective, cross 
section, and part plan of ceiling. It was sug- 
gested by the Renommee Hall, Liege, 
designed by M. Paul Jaspar. 
As shown, the main hall 
is 45 ft. wide, with side 
aisles and galleries all round, 
16 ft. 6 in. wide. The main 
hall is divided into three 
bays in the length, each 
being 45 ft. square and 
having a flat saucer dome, 
with ceiling light continuing 
the same curve, over. The 
saucer domes are of such 
a radius that if the curve 
is continued the diameter 
of the complete semicircle 
equals the diagonal of the 
bays. The main piers are 
connected longitudinally 
and transversely under the 
saucer domes by semi- 
circular arches. The saucer 
domes are ribbed and pan- 
elled, exhibiting the real construction, and 
the ribs, if continued in the same curve, abut 
either upon the semicircular arches men- 
tioned or upon the main piers, but below the 



ARCHITECTURAL AND SURFACE TREATMENT 



237 



level of the gallery ceiling, which is also the 
level of the flat ceiling of the hall. The 
panelling between the ribs is omitted ; in 
other words, we have what might be called 



open ribs we see the flat ceiling of the hall, 
which is, of course, of very small area, as 
nearly the whole area of the main hall ceiling 
is taken up by the domes. This is a treat- 




Fig. 4-14. Part Longitudinal Section of Exhibition Hall with Three Flat Saucer Domes 




Fig. 445. Part Plan of Exhibition Hall 

open pendentives, which give a fan-like effect ment which contains many suggestions 

to the ribs springing out of the main piers capable of further development. The whole 

and continuing those of and supporting the of the work shown is intended to be executed 

saucer domes. By looking up between these in reinforced concrete. 



238 



REINFORCED CONCRETE 



A design for a faade suitable for a club or 
similar building, to be erected in reinforced 
concrete, is shown in Figs. 449 to 452. The 
desire has been to produce a design as typical 
of the material as possible, and to base the 




Fig. 446. Interior View of Exhibition Hall 



the reinforcing rods in the flat roof turned 
up into it as shown, the whole being in 
monolithic connection. In order to obtain 
the effect of a frieze to the building, the 
filling-in walls to the top story have been 
brought out flush with the 
front of the piers, and the 
floor slab at third-floor level 
being brought beyond the face 
of the piers while the wall 
lintels are flush, the effect 
of a narrow architrave is 
obtained. The remarks that 
applied to the cornice apply 
also to the balcony at first- 
floor level. This has been 
shown with a panelled front 
in the same material, although, 
of course, a wrought-iron rail- 
ing would be just as suitable 
a finish to a reinforced con- 
crete balcony as to one in 
stone. In order to get some 
effective vertical shadows, the 
front has been treated with a 
feature at each end and a 



architectural treatment throughout on the 
structural features of the building. The 
cornice has been given a good projection, the 
blocks under it being merely the floor beams 



recessed cen- 
tral portion. 
The windows 
throughout are 







Fig. 447. Cross Section of Exhibition Hall through one 
of the Domes 




Fig. 448. Part Plan of Ceiling 
of Exhibition Hall 



continued through the wall, with brackets 
under. The necessity for the blocking being 
at the back of the cornice to weigh it down, 
as in masonry construction, having dis- 
appeared, it has been brought forward, and 



steel casements ; those to second floor, hav- 
ing no subsidiary piers to give depth to the 
reveals, have been given broad frames which 
lap over the concrete. The wide ground- 
floor windows to the end features are kept 






ARCHITECTURAL AND SURFACE TREATMENT 



239 



nearly flush with the back of the piers, so 
that good reveals are obtained. The rest 
of the ground-floor windows are also kept 
back, but small subsidiary piers are intro- 
duced, and the filling in is on the cant. The 
first- and second-floor windows to the end 
features are treated as flat bays, the filling 
in sloping from the small subsidiary piers 
to the back of the main piers. 

The remaining first-floor windows should 
have small subsidiary piers, which are 
treated externally as pilasters, with brackets 
and hoods ovelr. The hoods are brought 



out in a curve from the face of the wall, as 
is often done in roughcast, this seeming to 
suggest a plastic material. 

The filling-in, or mask, walls have been 
designed so as to suggest panelling, and the 
face of the wall lintels to the second floor 
has been kept back from the face of the 
piers and flush with the filling, so as to give 
more pleasing proportions. 

The exterior surface is intended to be 
finished without any applied ornament. 
The exterior of piers and beams would be 
finished with a smooth surface, obtained by 



-e 
U 





Fig. 451 Fig. 452 

Figs. 451 and 452. Detail of Reinforced Concrete Facade in Elevation 
and Vertical Section 



240 



ARCHITECTURAL AND SURFACE TREATMENT 



241 



ubbing down the surfaces after the removal 
if the moulds and floating them with a 
hin wash of cement and sand grout. The 
ixterior of panels and other sunk sur- 
ges would be treated by washing out the 
outer film of cement so as to expose the 
particles of stone used as matrix and aggre- 
;ate. 

As forming a contrast to the illustrations 
above referred to, attention will now be 
directed to Figs. 453 to 457, which, while 
scarcely typifying, undoubtedly suggest the 
place taken in the past by reinforced con- 
crete considered purely architecturally. The 



SURFACE TREATMENT 

The reason offered by architects why 
reinforced concrete is nearly always covered 
up is its alleged lack of inherent beauty 
as a building material. Fortunately, the 
general adoption, some time in the future, 
of more artistic methods of finishing will 
remove the reproach. Clearly, until rein- 
forced concrete is boldly exposed it will 
never be able to exert its legitimate influ- 
ence over external design, although, even 
as now used, it has had a certain influence 
on design, particularly in commercial archi- 
tecture. Let it be remembered that a build- 





Fig. 453. Small Hall with Mask Walls of Brick and Principals and Roof of Reinforced 

Concrete 



illustrations show a small public hall or 
parish hall apparently built of traditional 
materials, the exterior walls being in local 
bricks with red facings, and the interior 
walls being in Fletton bricks. These walls 
are merely mask walls, however, carry- 
ing no weight, and they could just as 
well, apart from aesthetic reasons, be of 
reinforced concrete 6 in. thick, finished, say, 
with roughcast. The roof and the princi- 
pals are of reinforced concrete, the con- 
struction being sufficiently shown by the 
cross section, a feature in which is the 
arched beam whose virtues have already 
been briefly noted. 



ing covered with stone will always be 
described by the general public as a stone 
building, of whatever material the backing 
may consist. 

The question, therefore, of the surface 
treatment of concrete is one of import- 
ance to the future of reinforced concrete, 
especially in view of the fact that the 
repeal of restrictive legislation now per- 
mits thin exterior walls (in other words, 
mask walls) to be employed. The subject 
of surface treatment has already received 
great attention, but the matter, is one for 
further experiment ; and in the search for 
more pleasing finishes those whose effect 






REINFORCED CONCRETE 



will be enhanced, not ruined, by the action By " natural finishes " are meant those 

of the weather the knowledge of the obtainable simply by varying the aggregate 

chemist, as well as of the practical man, and the nature of the facing in the form, the 

should be enlisted. Definite examples of surface left on removing the false -work 

the results obtained, as well as of the undergoing either no treatment at all or 





Fig. 455 



Figs. 454 to 457. 
Longitudinal Sec- 
tion, Plan, Front 
Elevation and Gross 
Section of Hall with 
Reinforced Concrete 
Principals and Roof 



J* 



1 



a 



Fig. 456 

methods ordinarily adopted, should be easily 
accessible for inspection by architects gener- 
ally. 

The methods of finishing concrete sur- 
faces will now be considered in detail. They 
may be classified under two main headings : 
(1) natural finishes, and (2) applied finishes. 




Fig. 457 

only sufficient to bring into relief the aggre- 
gate in the facing or to make the texture 
uniform. 

" Applied finishes " are those obtained 
by the application of plastic materials. 

The Untouched Surface. A common 
finish, but one that assthetically fails to. 



ARCHITECTURAL AND SURFACE TREATMENT 



243 



please many people, is to leave the wall just 
as it comes from the forms, with all the 
board marks showing ; but what it lacks in 
beauty it makes up in efficiency. C. K. 
Knapp, in a paper read before the National 
Association of Cement Users (U.S.A.), men- 
tioned an instance in which the original 
cement skin had been left undisturbed, and 
in which it defied for a number of years the 
penetrative power of water which lay in a 
pool over a ceiling. He stated that after 
protracted storms the surface seems to dry 
off instantly, while neighbouring houses of 
brick, and even frame construction, retain 
evidences of moisture upon the outside long 
after the concrete house has assumed its 
natural colour. Not only, said Mr. Knapp, 
was 40 to 60 saved for tool dressing, but, 
in the opinion of the architect, the finish 
was more artistic, as well as indicative of 
the plastic nature of concrete. 

It may be taken as granted, however, in 
spite of an occasional opinion to the con- 
trary, that the uninteresting nature of the 
undisturbed finish, its drabness, and its 
entire lack of light and shade effects, are 
obstacles to the employment of concrete in 
many quarters, and that something different 
will nearly always be demanded. 

The Brush Finish. One of the cheapest 
and most satisfactory methods of treating 
the green concrete is to give it a stiff brush- 
ing, taking care to make the effect as uniform 
as possible, it being quite easy, either with 
an ordinary scrubbing brush or with a wire 
brush, to vary the " pattern " as the work 
proceeds. 

It will be noted that the concrete must 
be green ; but it is not possible to give a 
definite time from the placing of the concrete 
to the commencement of the brushing, as 
naturally it will depend upon the propor- 
tions of the ingredients, the state of the 
weather, etc. Should the brushing be 
attempted before the concrete is sufficiently 
set, the result will be irregular, as any small 
aggregate present may easily be broken 
out ; on the other hand, should the brush- 
ing be delayed too long, the concrete may 
have become too hard to be treated by 
this method. 

The domestic scrubbing brush answers 
when the concrete is very green, and the 
wire brush when the concrete is harder. 
It is customary to flow water over the 
surface of the concrete freely during the 
course of the brushing, and this is un- 



doubtedly most conveniently done by 
means of a hose with a rose head. 

It is customary to assist the brushing by 
treating the washed surface with dilute 
muriatic or acetic acid, which helps to clean 
the aggregate and conduces to uniformity 
of texture throughout the whole job, which 
very possibly has been scrubbed down at 
varying intervals of time. One part of acid 
diluted with three parts of clean water is 
suitable ; but when the concrete has been 
made with white cement and white stone 
aggregate, it is better to use a 1 : 3 dilute 
sulphuric acid. 

An objection to the use of acids (muriatic, 
acetic, etc.) is the possibility of the forma- 
tion of stains and efflorescence, which 
certainly are very unsightly. An acid- 
treated surface needs to be well washed 
with plenty of water. 

The Carborundum Finish. The well- 
known carborundum finish gives a light- 
coloured surface and, by filling in the pores 
with cement, renders the material less 
pervious to water. Immediately the forms 
are removed, the surface is wetted and then 
rubbed with a No. 16 carborundum stone 
until a lather forms and extreme roughness 
has been removed. The work is washed 
down with the help of a brush, and while 
wet dusted with a 1 : 2 mixture of cement 
and fine sand, which is next rubbed in with 
a No. 16 stone, the finish being applied by 
rubbing with a No. 30 stone. A committee 
appointed by the National Association of 
Cement Users (U.S.A.) has recommended 
this method, but specified the use in the 
early stages of a No. 8 carborundum stone 
brick instead of the (finer) No. 16. 

The Sand-blast Finish. Sand-blasting 
has proved a quick and convenient method 
of finishing, the resultant surface rather 
resembling that produced by scrubbing, but 
being more regular. The principle is the 
abrasion of the surface by particles of sharp 
sand carried at a high velocity by a current 
of air under pressure. The abrasive removes 
the board marks and produces a uniform 
matt finish. 

Whilst practically any type of sand-blast 
apparatus can be adapted to the purpose, 
there is much to be said for the Niagara 
pattern illustrated by Fig. 458. This is a 
simple T-piece weighing about 6 lb., and 
it acts on a well-known principle exempli- 
fied by the familiar scent-sprayer. The air, 
under pressure of from 80 lb. to 90 lb. per 



244 



REINFORCED CONCRETE 



square inch, is supplied through the flexible 
l|-in. hose shown, and, in popular language, 
produces suction in the IJ-in. vertical pipe, 
whose lower end, weighted, is placed in the 




Fig. 458. T -piece for Sand-blast Apparatus 

sand. The sand, by the way, can be held 
in a vessel or may simply be in a heap. 
When the device needs cleaning, the nozzle 
is pressed against a hard surface, this 
causing the compressed air to find an outlet 
through the suction pipe and effectually 
clearing this of any obstruction. Obviously, 
any similar material to sand can be used with 
the device, and it may sometimes be an 
advantage to use something with less pro- 
nounced cutting properties. 

Bush-hammered Finish. What is 
known as "bush hammering " so-called 
from the use of the mason's bush hammer, 
one form of which tool is illustrated by 
Fig. 459 destroys the facing film of cement, 
and therefore injuriously affects the water- 
tightness of the work ; but where this is un- 
important, the method can be recommended 
as producing an interesting surface, the 
cement between the aggregate being chipped 
out and the stone aggregate itself being 
roughened. The work should be done within 




Fig. 459. A Type of Bush Hammer 

about two months of setting, as otherwise 
the concrete may be too hard to be affected 
by the tool. Stone masons' tools of the axe 
and patent-axe type will be familiar to most 
readers ; the bush hammer is used in prac- 



tically the same way, either by hand or in a 
compressed-air machine tool. 

Facing Concrete in the Form. The 
full advantage of most of the above methods 
of finishing can only be realised when special 
precautions have been taken to see that the 
work is given a facing at least 1 in. thick- 
of fine material in the form ; an applied 
facing, plastered on the hard concrete, is 
not the same thing. The facing is applied 
in course of executing the main concreting 
by plastering it on the form and then 
introducing the body concrete ; or, in- 
stead, the body concrete is inserted anc 
then pushed back from the form to allow of 
the fine stuff being introduced ; still another 
method is to use iron plates, as described 
later. By "fine stuff" is meant a mortar 
or concrete with a small aggregate suitable 
to show on the face of the work ; it certainly 
does not mean excessively rich stuff, as this 
is always liable to go " crazy," that is, 
develop hair cracks all over its surface. 
1 of cement to from 2 to 3 of fine aggregate 
is a suitable proportion. When the facing 
is to contain both fine and coarse aggregates, 
suitable proportions are 1 : 1J : 3, 1:2:3, 
or even 1:2:4, the ingredients being 
cement, sand or stone screenings and crushed 
stone or screened gravel, and the facing must 
be twice as thick as the diameter of the 
largest stone in the aggregate. 

To facilitate the placing of both the facing 
mixture and the backing without mutual 
interference, the use of iron plates " grano- 
lithic plates " is recommended. These maj 
be of any suitable depth and length, 12 in. 
X 5 ft. or 6 ft. being suitable, furnishec 
with a handle near each end and flared oi 
at the top, as in Figs. 460 to 462. On the side 
opposite to the flare two or three angle-irons 
or T-irons are riveted to act as distance 
pieces. As many of these plates as requirec 
are put into the forms, their ends slightlj 
overlapping, with the angles or T's close to 
the face of the form, in this way forming 
a deep, narrow slot into which the facing 
stuff can easily be poured, the flaring of 
the plate assisting this. The concrete back- 
ing is poured in at the same time, but the 
facing mixture is kept at a slightly higher 
level to prevent any thin stuff from the 
concrete running over the top of the facing 
and finding its way to the face of the form. 
When the layer is of the desired thickness, 
the plates are raised in readiness for the 
next layer, and the two materials the bod) 



ARCHITECTURAL AND SURFACE TREATMENT 



245 



concrete and the facing brought into 
intimate union by ramming. 

When the aggregate in the facing material 
has been selected especially to give character 
to the concrete, care must be taken to ensure 
its fineness, and it will be necessary to re- 
move the cement skin from the face of the 
work. It is usual to use as aggregate stone 
or pebbles crushed to pass through, say, a 
-in. screen, but to be retained upon a |-in. 
screen. An average diameter of f in. is 
about right. The fine stuff is applied to the 
face of the form by means of a trowel or 
plasterer's float just before introducing the 
body concrete ; or the " granolithic plate " 
method already described is employed. Care 
must be taken to get the two materials 
into intimate union. By means of a little 
thought it is possible to choose aggregates 



a wooden block, or with sandstone and 
plenty of water, but this method will not 
leave the aggregate in relief. Again, bush 
hammering, as already described, may be 
adopted. The following suggestions are due 
to Henry H. Quimby, M.Am.Soc.C.E. : " If 
the height of the wall to be thus treated is 
too great to be completed in one day, face 
forms must be constructed to facilitate the 
removal of the planking without disturbing 
the studs or uprights. This is easily accom- 
plished by setting the studs 8 in. to 12 in. 
away from the face line and supporting 
planks with cleats say 2-in. by 1-in. 
tacked to the studs and the planks. This 
permits the lower planks to be removed and 
the washing done while the upper planks 
are in place and concrete is being deposited. 
With the exercise of very watchful care on 



fit. 460 



Fig. 461 



[ C 



Fig. 462 
Figs. 460 to 462. Front and End Elevations and Plan of "Granolithic Plate" 



of different colours and sizes to produce 
excellent effects, and a note may here be 
made of the suitability of selected ground 
mica for this purpose, this material supply- 
ing " life " and sparkle suggesting freshly 
wrought granite. The next part of the pro- 
cedure will depend largely upon the nature 
of the work and the setting power of the 
cement used. All that can here be said is 
that the surface of the granolithic con- 
crete must be exposed while the material 
is still friable, so that an immediate wash- 
ing with water and a stiff brush will remove 
the cement film and expose the aggregate, 
which will now appear in decided relief 
and of a rough, coarse texture. (The illus- 
trations, Figs. 463 to 468, on later pages 
are full-size photographic reproductions of 
patterns obtained by the above method, 
and the inscriptions give all necessary 
particulars. They are due originally to 
Engineering Neivs") Even when the face has 
become too hard for brushing, something of 
the effect can be produced by rubbing with 



the part of the workmen and unremitting 
inspection, two different days' work can be 
joined so that, after washing, the joint will 
not be unsightly even scarcely distinguish- 
able ; but such work is usually not obtain- 
able throughout a structure, and it is found 
very easy to obtain thoroughly satisfactory 
joints by indenting horizontal grooves at 
regular intervals representing courses, and 
finishing each day's work at the apex of 
a groove. These indentations are made by 
means of triangular beads on the face forms. 
Usually the bead is the bevelled edge of a 
strip set between the face planks and lightly 
secured to the planks with partly driven 
toe nails, so that, if desired, a plank can be 
removed independently of the bead above 
it, the bead remaining to set the plank upon 
the next course. These grooves in the face 
of a wall improve the appearance by re- 
lieving the blankness of a large area. It is 
found practicable to prosecute the work 
with one course of planks where the capacity 
of the plant for one day is equal to only one 



246 



REINFORCED CONCRETE 



course of concrete. In this way the same 
planks have been used for many different 
courses on four or more different structures." 




Fig. 463. Sand Concrete, 1 : 2. Full size 



Uniformity of aggregate, 
both as regards size and 
colour, is of great import- 
ance in this method of 
finishing. The matter needs 
careful attention before 
any work is started in 
order to minimise the risk 
of being obliged to com- 
plete the walling with 
aggregate of a different 
character. 

The nature of the tamp- 
ing exercises a great influ- 
ence on the texture of the 
face of the work. For 
example, when a large 
aggregate is wished to 
show in the face of a wall, 
the tamping should be done 
in the middle, this carry- 
ing the stones against the 
form. Tamping near the 
side helps in getting the 
finer stuff towards the face. 
In producing horizontal, 
flat surface, ,, skilful tamp- 



ing will bring the finer stuff to the top, 
but a special tamper the " Andrews," of 
American origin, but obtainable in both the 
United States and Great 
Britain has been intro- 
duced to facilitate this 
particular work. As shown 
on p. 129, the tamper 
has a number of pyramidal 
points on its working sur- 
face, so arranged that 
when a mass of concrete 
containing large particles 
of stone and gravel is 
tamped, these larger par- 
ticles are pushed down 
further into the mass, and 
the thin stuff flows to the 
surface. The tool is 8 in. 
square, and the points 
projecting from the face 
are connected at their 
bases in such a way as to 
prevent particles of stone 
from becoming jammed 
between them. In use, 
the tamper is employed 
after the layer of concrete 
has been spread and 
roughly levelled. After its 




Fig. 464. Crushed Stone Concrete (Cement 1, Yellow Bank 
Sand 2, and f in. Screened Stone 3). Full size 



ARCHITECTURAL AND SURFACE TREATMENT 



247 



use, the surface may be smoothed off and 
finished like any other finish dressing. 
To cause the thin stuff in the wet mixture 







Fig. 465. Pebble Concrete with Scrubbed Surface (Cement 1, 
Bar Sand 2, and ^ in. White Pebbles 3). Full size 



to flow to the side of the 
form, and thus obtain a 
fine vertical facing with- 
out introducing a second 
mixture, the use of a per- 
forated spade (see p. 128) 
has been suggested ; and 
many contractors will find 
it convenient to carry out 
experiments with the ob- 
ject of applying the idea 
in other ways. A spade 
illustrated on p. 129 is 
that known as the Ross. 
Its action is to force back 
the coarser aggregate and 
to allow the finer stuff to 
come to the face of the 
work. Other tools for a 
similar purpose are illus- 
trated on pp. 128 and 129. 
Pebble Dashing. To 
imitate the effect of stucco 
upon which pebbles have 
been dashed, the face of 
the form is plastered with 



well-worked wet clay into which have been 
lightly pressed small pebbles, pieces of glass 
or glazed tile, marble chips, etc. When, 
after the concrete has set, 
the form is taken away, 
the clay is washed off and 
the surface brushed to re- 
veal the pebbles, etc., in 
the face of the concrete. 

Sand Finish. The me- 
thod is on the lines of that 
described in the preceding 
paragraph. Clay is well 
tempered and thoroughly 
worked, and then plas- 
tered on the inside of the 
forms, working it, if de- 
sired, into patterns. Sand 
is applied evenly to the 
wet clay, and then the 
concrete is poured in. On 
the removal of the forms, 
the clay is washed off with 
brush and water, the sand 
adhering to the concrete. 
This method certainly ap- 
pears to be unnecessarily 
troublesome. Merely to 
get a sand finish on plain, 
flat walling, it should 
suffice to .brush over the 



' *' KLI ~ 

mm 



f& .'> ' Y* 1 

A'I'O -r 

Af\*4&;.> &r$ai 



Nt^-C?^, 

so? 






&- 






^M 

Fig. 466. Granite Grit Concrete (Cement 1, Bar Sand 2, and 
i in. Granite Grit 3). Full size 



REINFORCED CONCRETE 



form with clay water, and apply the sand, 
which will readily adhere. 

Glazed Finish. Whilst concrete cannot 
be truly polished, it is not difficult in theory 
to obtain on it a glazed surface. All that 
would be necessary would be to line the 
form where required with well-cleaned and 
polished glass, or, as an American experi- 
menter has proposed, with enamelled iron 
plates, of the kind universally employed for 
large advertising signs. In practice, the 
use of such an untractable and brittle 
material as glass would cause never-ending 
difficulty, although it might be possible 
without much inconvenience to introduce 
narrow slips for the purpose of casting 
ornamental devices on the work. The 
employment of steel forms stove-enamelled 
on the working side might provide a solution 
to the problem should the production of 
glazed surfaces be demanded. A practical 
difficulty would be the almost inevitable 
formation on the form of air-bells, as these 
would, of course, spoil the surface ; but the 
trouble might be got over by first lining the 
form with fine cement mortar \ in. or less 
in thickness, applying it with heavy pres- 
sure. It is reasonable to assume that the 
polished skin would give the concrete more 
than usual resistance to weather and to 
mechanical wear. 

APPLIED FINISHES 

Plastered Surfaces. When a concrete 
surface is finished by coating with plaster 
of whatever nature, there is always a risk 
of this coming away at a later date should 
there be a likelihood of moisture percolating 
through the concrete and affecting the back 
of the plaster. However, plastering is often 
successful, but much depends upon the pre- 
paration of the concrete surface and the 
kind of plaster used. A good plan is to 
wash the concrete with dilute acid, talcing 
care to rinse this off afterwards, and then, 
while the surface is damp, but not flooded 
with water, to apply the plaster. The 
moisture in the surface of the concrete 
assists the cement mortar to set. The 
following method, advanced by an American 
contractor, should give good results. The 
first coat is a 1 : 3 mixture as dry as it can 
be applied, this necessitating considerable 
pressure to cause it to adhere well. Let 
the coat be thin and scratch it afterwards. 
Brush over the first coat with water and 
apply the second coat, using as much pres- 



sure as possible. If a third coat is neces- 
sary, adopt the same precautions. 

A concrete surface gives a better key for 
plaster after it has been hacked over with 
axe or bush hammer, but this necessitates a 
thick coat of plaster. If the concrete is soft 
enough to be affected by a coarse wire 
brush, a rub down with that will answer 
instead of hacking, and the plaster coat 
can then be thin. The acid treatment and 
washing, as described on p. 243, should 
precede the plastering. 

Tiles, Mosaic, Sgraffito, etc. In some 
situations, tile, or the old Byzantine material, 
vitrified mosaic, makes a beautiful and suit- 
able finish. In either case, they would have 
one constructional advantage, especially in 
high buildings, over brick or masonry 
casings, in that the weight added by them 
to the foundations would be comparatively 
trifling. A concrete wall having a true, flat 
surface could be prepared for tiling by 
rubbing down with a carborundum stone and 
then scratching with a brush bristle or wire, 
according to the age of the concrete ; the 
wall should be wetted and the tiles bedded 
in neat cement, using as small a quantity 
as possible to get a good result. Much the 
same instruction applies to the mosaic. 

It is somewhat surprising that sgraffito is 
not more generally adopted as a finish to 
concrete work where ornamental effects are 
desired ; certainly in some cases it might 
be very effectively employed. Sgraffito is 
an Italian word meaning "scratched," and 
is the name of a very old process consisting 
in applying to the surface several coats of 
different colours, these coats being brought 
to view as required by chipping or scraping. 
In the simplest form there are but two 
coats black for the ground, white for the 
covering ; and the surface to be treated is 
floated to a uniform face, allowing for the 
finishing coat, which will be in. thick. 
The outline of the design is marked on the 
face of the floating, as a guide for keying 
the J-in. finishing coat, which should be 
applied as soon as the colour coat is suffici- 
ently hard. Placing the design drawn on 
paper in its original position, pounce through 
the outline as a guide for cutting through 
to the colour coat, using a worn knife to 
cut away the superfluous material, which is 
removed with a spatula ; the edges of the 
work being sloped or inclined according to 
the light or shade required. For three-coat 
work the colour coat may be left rough, 



ARCHITECTURAL AND SURFACE TREATMENT 



249 



providing it is uniform, a dark colour being 
used as a background. This coat is ruled 
to within f in. of the finished face, the class 




Wjf* . 




Screened Yellow Pebbles 3). pull 



tained by adding a sample of it to a small 
quantity of the stuff with which it is to 
be used, the material being allowed full 
time for setting. For ex- 
^HH^HI ternal work, portland ce- 
ment or Aberthaw lime 
may be used, in the pro- 
portion of 2 parts sand to 
1 part cement. Aberthaw 
lime may also be used for 
internal work, and so also 
may Parian cement ; but 
for ordinary purposes se- 
lenitic may be used with 
satisfactory results. 

Fig. 469 (p. 251) shows 
the excellent effect ob- 
tained by the adoption of 
inlaid faience tiles and red 
bricks, the roof covering 
consisting of terra-cotta 
tiles. 

"Stuc" Work. This 
is a form of interior treat- 
ment particularly suitable 
for application to concrete 
walls and lending itself to 
a simple and effective de- 
sign. In appearance, it 
suggests a good Bath stone. 
The material is made with 




Sand 2, and 
size 



of work determining the. 
thickness of the various 
coats. Apply the succeed- 
ing coat, and finish as 
soon as convenient, so that 
the coats may adhere in 
one compact mass. Expe- 
dition is specially neces- 
sary when treating exterior 
work, as water settling on 
the incisions of work im- 
properly keyed would cause 
it to laminate or scale off. 
For colouring matter, to 
obtain good black use 
bone-black or, for ordin- 
ary work, smiths' ashes 
as an aggregate ; for red, 
Venetian or Indian red ; 
for brown, umber ; for 
yellow, yellow ochre. When 
a neutral tint is required, 
a combination of two or 
more colours may be em- 
ployed. The depth of the 
colour should be ascer- 




Fig. 468. Sand Concrete, 1 : 3 



250 



REINFORCED CONCRETE 



a mixture of plaster-of-paris, cement, size and 
various colouring matters, the latter vary- 
ing slightly according to the desired finished 
effect. In the application, a very moist solu- 
tion of plaster-of-paris is first stippled on the 
walls with a stiff broom to give a key for the 
" stuc." The plaster and cement are then 
placed in a mixing box about 2 ft. by 1 ft. 5 in. 
and about 12 in. deep, and thoroughly 
mixed. When this is complete, the size, 
water and colours are added, and the whole 
thoroughly incorporated, and the mass is 
squeezed through the hands until a thick 
cream is obtained. The resulting compound 
is very quick in setting, and consequently 
it requires to be rapidly applied to the sur- 
faces, these having been previously screeded. 
The hands and a brass trowel are used, the 
plaster being literally thrown on. Whilst 
moist, it is roughly lined to the screeds, and 
the surfaces are planed and scraped to a 
fairly even and true face. When set, it is 
left for about two months, and during this 
period various stains come out to the face 
of the " stuc," which assumes a light brown 
colour, and becomes very hard. After this 
time a scraping plane is employed and the 
whole of the outer surface is removed, taking 
away all stains and leaving a fine soft stone 
effect. During the scraping process, small 
pit holes are formed in the surface, which 
greatly improves the appearance, and re- 
moves the perfectly smooth and artificial 
effect otherwise given. The whole surface 
is then rubbed down with pumicestone, and 
the various joints are set out with chalked 
cords in accordance with the design. These 
lines are then cut in and filled up with white 
plaster, and a final rubbing down with 
pumicestone is given. Mouldings and cor- 
nices are run in the orthodox manner with 
a " horse," several plasterers throwing on 
the material at each point, whilst the horse 
is worked backwards and forwards. 

When using the " stuc " on concrete work 
it is advisable to give the walls a thin coat- 
ing of Keene's cement to prevent the staining 
of the " stuc." 

In the execution of the ornament, some 
excellent work is seen, as this is not cast, 
but modelled in position by the plasterer, 
and for this reason every man requires to be 
an artist fully in sympathy with his work. 
" Stuc " work was extensively adopted in the 
decoration of the Koyal Automobile Club, 
and it can be said confidently that the work 
to the fluted columns round the gallery and 



the Eoman-Doric caps, with the enriched 
frieze above, are greatly superior to similar 
work executed in ordinary fibrous plaster. 
The " stuc " work requires neither painting 
nor other finish, and retains its fine appear- 
ance if merely rubbed down once every three 
years. 

COLOURING CONCRETE 

In response to a demand for a brighter and 
more interesting colour than that provided 
by natural concrete, there has been a great 
deal of experiment to determine the best 
methods by which the desired result can be 
obtained. Undoubtedly, the most artistic 
method, but one not suited to all or even 
the majority of situations, is to determine 
the colour by the use of a specially selected 
aggregate which will give, not only colour, 
but texture to the work. Sufficient has been 
said on this subject earlier in this chapter. 
Another method is to face the walls with 
tiles, mosaic, etc., as already described, but 
this is expensive, and has the disadvantage 
of masking the character of the concrete 
surface, and in that particular sense is false 
art. Strictly, methods of colouring concrete 
include only those by which (a) pigment is 
incorporated with the concrete or with the 
facing material introduced into the form, 
(6) staining or (c) distempering, or (d) oil- 
painting the concrete surface. 

Body Colours. But for the fact that the 
lime in the concrete has an injurious effect 
upon the majority of pigments, there would 
be very little to say under this heading. 
This injurious action has long been known, 
and everyone is familiar with the bleaching 
of paint and wallpaper by the lime in newly- 
plastered walls. The vegetable and many 
of the artificial pigments are the worst 
offenders, leaving the selection between the 
relatively few mineral pigments. A com- 
mittee appointed by the National Association 
of Cement Users (U.S.A.) has reported that 
the only mineral pigments that should be 
used are comprised in the following list : 
Lampblack, manganese dioxide, red iron 
oxide, English red oxide, brown roasted iron 
oxide, brown ochre, yellow ochre, ultra- 
marine, chromium oxide, ultramarine green 
and violet oxide of iron. This brief list of 
pigments does not allow of much latitude. 
The committee further reports that the 
amount that can be safely used is small 
owing to the danger of impairing the strength 
of the concrete. Five per cent, by weight 




251 



252 



REINFORCED CONCRETE 



to that of the cement should be the limit, 
beyond which the impairment of strength 
is too great to justify a larger amount, 
except in special circumstances, as, for 
example, in a thin facing. Even this pro- 
portion of those pigments whose colour 
differs little from that of the cement does 
not produce a marked change. Lampblack, 
it is pointed out, is the principal pigment 
used for darkening, on account of its strong 
contrast and the fineness of its particles. 
For lightening the colour somewhat lime is 
the best material to use. Colouring matters 
can be used in either a dry form or as a 
paste. It will generally be found most 
convenient for mixing to use the dry form, 
thoroughly mixing it with dry mortar before 
the addition of any water. 

The pigments exercise an influence on 
the setting properties. Crimson lake (alum- 
ina base) and barium chromate quicken the 
cement ; manganese dioxide, red ochre, and 
chinese red retard it ; and ferric oxide, 
yellow ochre, ultramarine, and chromium 
oxide have a slight quickening effect. 

In colouring concrete by the admixture of 
pigment, the resultant tint must not be 
judged by that of the wet mixture, the con- 
crete always drying out lighter than that 
would suggest. The best course is to mix 
together small measured quantities of all 
the ingredients dry, add the water, thor- 
oughly incorporate, and allow a portion to 
set in a mould ; the experiment can be 
repeated until the desired result is obtained. 

For a white concrete, use 1 of white port- 
land cement and 2 of hard marble, screening 
to pass through a No. 8 screen and be col- 
lected on a No. 40 screen. When the con- 
crete is hard, in three or four days, it may 
be rubbed with a terrazzo stone to polish 
the marble particles. 

Before giving a table of concrete colouring 
pigments, it is desirable to summarise the 
experiments carried out in the United States 
by Prof. Charles E. Pellew with the object 
of finding the most suitable pigments for 
colouring concrete. The question of expense 
was of vital importance, and in the matter 
of first cost it was evident that surface 
colouring would be cheaper. But, unless 
there is a thoroughly hard permanent sur- 
face for the pigments to adhere to, and 
to obviate as far as possible the use of 
linseed oil, upon which the lime exerts a 
strong action, the body colouring is pro- 
bably the most satisfactory. For yellow, 



Prof. Pellew thinks that the only available 
pigment is yellow ochre, 8 per cent, of a 
strong pigment giving a bright tan colour. 
This yellow can be used for shading the red, 
but is not so effective for this as the man- 
ganese brown. Yellow ochre with small 
amounts of permanganate brown will give 
various shades of yellowish brown or buff 
colour. BlacJc is obtained by using carbon 
black or lampblack. For a bluish shade a 
black iron oxide, imported for the use of 
gas works, gave satisfaction. The only red 
pigments available are red oxides of iron, 
some of them natural, finely ground haema- 
tites, and others artificial. They differ 
greatly in shade, price, and staining power, 
the cheaper pigments being unsatisfactory. 
It is best to use a small amount of a strong 
though high-priced pigment than larger 
quantities of a weak but cheap one. After 
experimenting with twenty or more different 
colours from various manufacturers, the 
best results were obtained from a red colour 
at five cents (2|d.) a pound ; from 7J to 
10 per cent, (of the weight of cement) was 
needed to give a full shade. A slight addi- 
tion of permanganate brown gave a red 
terra-cotta shade. As regards brown, a 
vegetable pigment proved unsatisfactory. 
Prof. Pellew tried iron-rust, the action of 
which is based on the formation in the 
concrete of a reddish-brown deposit of ferric 
hydroxide by the action of the lime of the 
cement on a soluble salt of iron, like ferric 
chloride or ferric sulphate. Unfortunately, 
it takes 25 to 30 per cent, of ferric sulphate 
(of the weight of concrete) to get at all a 
decided colour with this compound, and this 
is a serious drawback. He tried manganese 
brown, which is based on the formation in 
the concrete of brown manganese hydroxide 
by the reduction of the salt potassium per- 
manganate. The latter possesses a strong 
rich purple colour, which in the presence of 
oxi disable material, such as organic matter, 
turns at once to a full seal brown. In stain- 
ing concrete the organic matter must be 
supplied in the form of glucose or sugar, 
which in quite small quantities will change 
the deep purple colour of the permanganate 
into a rich seal brown. To get a deeper 
colour, Prof. Pellew was obliged to use 
24| per cent, (by weight of the cement) of 
permanganate and about \ per cent, of 
glucose. With regard to a green colour the 
high price of chromium oxide, the only green 
mineral pigment that will stand the action 



ARCHITECTURAL AND SURFACE TREATMENT 



253 



of lime, prevents its general use. In the 
absence of a strong blue that will stand the 
action of lime, it is not possible to obtain 
a good green by modifying the colour pro- 
duced by yellow ochre. Ultramarine blue 
is fast to lime, but its staining power is low 
when mixed with other pigments, while 
Prussian blue is easily attacked by the 
cement, and is not, therefore, included in 
the table of pigments given on this page. 
It seems probable that for greens some 
form of surface colouring will have to 
answer, but much remains to be done in 
the way of experiment. 

TABLE OF PIGMENTS 

The weights of pigments given in the 
following table are those recommended for 
adding to each cubic foot, reckoned as 90 lb., 
of cement ; but the table must not be taken 
quite literally, as experiments have obtained 
widely different results. No greater claim 
can be made for the table than that it will 
act as a rough guide. It cannot do more, 
owing to the great differences frequently 
existing between two pigments passing under 
the same name. They may be unlike one 
another chemically, physically and in their 
percentage content of inert cheapener which 
so seriously affects the staining power. The 
pigments mentioned will certainly give the 
tints, but the proportions must be adapted 
as required, depending, as above suggested, 
on the strength of the pigments them- 
selves and also on the proportions of the 
concrete. 

BLACK . . 10 lb. of manganese di- 
oxide or lampblack. 

, BLUISH . 8 lb. or 9 lb. of black iron 
oxide. 

BLUE . . . 3 lb. to 4| lb. of ultra- 
marine. 

, VIOLET . 4J lb. of violet oxide of 

iron. 

BROWN . . 4 lb. of brown ochre. 

, CHOCOLATE 6 lb. of manganese di- 
oxide, 4 lb. of red oxide 
of iron, and 2 lb. of 
black oxide of iron or 
copper ; or 3| lb. of 
burnt umber. 

BUFF . . . 3J lb. of yellow ochre ; or 
4J lb. of yellow ochre 
and 2 lb. of perman- 
ganate brown. 

GREEN . . 10 lb. of chromium oxide 
or ultramarine green. 



GREY 

PINK . 
, DULL 



-, BRIGHT 
-, DULL 



SLATE 



-, BLUE 
-, PINKISH 



TAN . 

TERRA-COTTA 



YELLOW 
WHITE 



1 lb. to 3J lb. of 
black or manganese 
dioxide. 

3 lb. of crimson lake 
(alumina base). 

2 lb. to 3 lb. of Venetian 
red. 

| lb. of burnt umber. 

2 lb. to 3J lb. of Chatta- 
nooga iron ore or red 
iron ore. 

5 lb. of Pompeiian or 
English red. 

5 lb. raw iron dioxide ; 
carbon black may be 
added. 

| lb. to 3 lb. of lamp- 
black or manganese 
dioxide. 

1 lb. to 31 lb. of ultra- 
marine blue. 

| lb. to 1 lb. of Venetian 
red ; or lb. of Chatta- 
nooga iron ore or red 
iron ore. 

7 lb. of yellow ochre. 

English red with per- 
manganate brown ; or 
2 lb. to 31 lb. of Chat- 
tanooga iron ore or red 
iron ore. 

5 lb. to 10 lb. of yellow 
ochre. 

44 Jib. of barytes (barium 
sulphate). 



Stains. Manufacturers have placed upon 
the market a number of stains suitable for 
use on concrete. A brown stain is easily 
made by dissolving iron sulphate (green 
copperas) in water, the proportion being 
about 2^ lb. per gal. The colour is strength- 
ened by repeating the application. The 
addition of alum to the above solution gives 
a pale yellow ; whilst chrome alum gives a 
green. Stains have the advantage over 
paints that they do not cover the concrete 
with a thick coating, but sink into the pores 
and preserve the natural texture, while at 
the same time colouring it in rich, deep tones. 
They can be applied easily and rapidly, their 
covering power is two or three times that 
of oil paint, and they cost, bulk for bulk, 
much less. 

Distempering. Concrete can be suc- 
cessfully distempered with common white- 
wash or with cement washes ; as regards 



254 



REINFORCED CONCRETE 



colours, the pigments used must be fast to 
lime (see under the heading " Body 
Colours"). It will be better to kill the 
alkali in the concrete surface with dilute 
acid and then well wash with water before 
applying the distemper. 
' Painting. By this term, of course, " oil 
painting " is understood. Painting is desir- 
able only on smooth concrete, the con- 
structional nature of which it is not desired 
to emphasise. The paint forms a film of 
metallic oxides, sulphates, etc., and oxidised 
linseed oil on the face of the work, closing 
the pores and undoubtedly adding to the 
weather-resisting properties. Unfortunately, 
the lime exerts a violent chemical action 
upon linseed oil, and special preparation of 
the concrete surface is absolutely essential. 
Usually, this preparation consists in killing 
the alkali and then coating with at least 
two coats of good white-lead paint before 
applying the finishing coat. But a different 
style of treatment was suggested some time 
ago before the American Society for Testing 
Materials by Charles Macnichol, who spoke 
from the experience of many years. He 
primes the concrete with zinc sulphate 
ground up with an equal quantity of water. 
Then, after two or three days, the concrete 
can be painted on safely. Dr. A. S. Gush- 
man suggests the explanation that a chemical 
reaction results in the formation of gypsum 
(calcium sulphate) and zinc hydroxide 
(hydrated oxide of zinc), which substances, 
held within the pores of the cement, do not 
affect linseed oil injuriously indeed, they 
are common paint pigments. The method 
is economical as regards the after coats, 
because the suction of the cement has been 
already satisfied. 

The architect will usually hesitate to 
specify any particular formula to be fol- 
lowed in painting concrete, and will generally 
content himself by specifying one of the 
several compositions on the market that are 
specially made for the purpose, the con- 
stituents of which are, for obvious reasons, 
kept secret. Some of them are the result 
of very careful trials, and Dr. Macmimilian 
Toch, in a lecture delivered some time ago 
before the Paint and Varnish Society, 
hinted that the best results might be ob- 
tained by a proper admixture of Menhaden 
fish oil and tung or Chinese wood oil. What- 
ever paint or preparation is used, it is 
generally recognised that it is useless to 
apply it to green or fresh concrete, but an 



exposure of at least a year, or even two years, 
will render the surface fit for receiving the 
paint. In actual practice, however, it is 
usually desired to finish the building almost 
immediately after it is erected, and in such 
a case it becomes necessary to prepare the 
surface in order that the paint may per- 
manently adhere. Even when the concrete 
has been exposed for a year or more the 
application of such a preparation may be 
made as a matter of precaution to neutralise 
any free lime which might remain. 

Opinion widely differs as to the com- 
position of this preparation. Many practical 
men advocate the use of a solution of zinc 
sulphate, commonly known as " white 
vitriol." This is used in the proportion of 
8 Ib. to 1 gallon of water. It is urged against 
this method that something more than a 
neutralisation of the free lime is required, 
and that to obtain satisfactory results a 
material should be used which will fill up 
the pores of the concrete and prevent 
suction. Hence there are advocates of 
compositions in which paraffin wax plays 
an important part, a method, it may be 
remarked in passing, which was successfully 
employed a year or so back in connection 
with Cleopatra's Needle on the Thames 
Embankment. 

Under ordinary conditions, however, the 
most practical method is to apply a liquid 
to neutralise the free lime, and, that object 
being thoroughly effected, the paint might 
be ordinary white-lead, although much 
better results would be obtained by the 
use of one of the special " concrete paints ' r 
already referred to. 

Among the solutions that have been 
recommended for use under the coating of 
paint are the following : For concrete floors 
the use of sodium silicate has been suggested. 
Dr. M. Bennett Blackler advocates the use 
of copals made up with a small quantity 
of oil and diluted with turpentine, and he 
expresses his opinion, also, that casein 
solutions treated when dry with formaldehyde 
would give a coat which is absolutely im- 
permeable to water and form a surface 
on which paint would stand. In these 
opinions Dr. Blackler has been backed up 
by many other eminent men ; in fact, it is 
not improbable that casein forms the active 
base of many of the specialities now on 
the market. 

Assuming that the surface is in as good 1 
a condition to receive the paint as can be 



ARCHITECTURAL AND SURFACE TREATMENT 



255 



expected, the next question arises as to 
what is the best paint to apply. Many 
people believe that when the walls are in 
a proper condition the ordinary paint can 
be used without difficulty, and the following 
formula has been published : For the 
priming coat : 100 Ib. of pure white-lead, 
9 gals, of pure boiled linseed oil, and 1 gal. 
of turpentine. This may be followed by 
using instead of the boiled oil 9 gals, of 
raw linseed, and 1J pints of turpentine 
drier. Another priming coat recommended 
consists of 85 Ib. of pure dry red-lead mixed 
with 1 gal. of boiled linseed oil, and \ gal. 
of turpentine. The body coat should in 
this case be 100 Ib. of pure white-lead, 
4 gals, of pure linseed oil (one third boiled 
and two-thirds raw), and the finishing coat 
100 Ib. pure white-lead, 3| gals, of pure 
linseed oil (one-third boiled, and two-thirds 
raw), and 1 pint of pure turpentine. The 
red -lead in the priming coat above mentioned 
would probably be in most cases objection- 
able because of its colour. 

When the surface of concrete is covered 
with cement, the latter may be painted by 
giving at first two coats of Alabastine, pro- 
ceeding with oil paint in the ordinary way. 
W. G. Scott recommends as a paint for 
cement the follow ing : 10 Ib. plaster -of - 
paris, 2 Ib. portland cement, 8 Ib. whiting. 
4 Ib. wheat flour, 6 Ib. zinc oxide, and 2 Ib. 
red-lead. This mixture should be ground 
in 1 gal. of prepared wood oil, and J gal. 
of soya bean oil. The pigments are mixed 
separately with the oil, adding them in the 
order given, and if the paste is then found 
to be too stiff a little more of the bean oil 
is added. This paste, Mr. Scott says, grinds 
with some difficulty, and it is better to mull 
or run through an iron mill first, and then 
to grind fine in a stone mill. In use, the 
paint is thinned with benzine or turpentine, 
or with a mixture of 7 parts of turpentine 
and 1 of paraffin oil. The prepared wood 
oil referred to may be made by stirring 
9 Ib. litharge and 1 Ib. of manganese borate 
into 50 gals, of Chinese wood oil, and heating 
for five hours at a temperature of 350 F. 
(177 C.). After the litharge and manganese 
borate have settled out and the oil is clear, 
it is ready for use. This paint or cement is 
intended for application to porous cement, 



and may be used on concrete. The same 
author gives the composition of a marine 
paint which will set under water, and which 
may be quoted here : 10 Ib. portland 
cement, 5 Ib. silicate, 3lb. zinc oxide, 7J Ib. 
red-lead, 5 Ib. litharge, and 2 Ib. graphite. 
These are mixed with 1 gal. of boiled linseed 
oil and ^ gal. of paraffin oil. It sets under 
water, and becomes in time as hard as 
stone. The same mixture if thickened with 
whiting or plaster-of-paris forms a valuable 
cement for many purposes. 

Some careful experiments conducted by 
Mr. Henry Gardner, assistant director of 
the Institute of Industrial Research, Wash- 
ington, D.C., are reported in his " Paint 
Technology and Tests." He carried out 
his tests on mortar made of 1 part of port- 
land cement, and 3 parts of sharp clean 
sand. Omitting the failures, the results 
were as follow : No. 1. Concrete primed 
with a 25 per cent, solution of zinc sulphate 
crystals dissolved in water. A wide brush 
was used for the application, and the spread- 
ing rate was approximately 200 square feet 
per gallon. Second and third coated on 
the second day with the following com- 
position : Sublimed white-lead (similar to 
" Purex " in Great Britain) 50 per cent., 
zinc oxide 35 per cent., silica and baryte& 
12 per cent, and Prussian blue 3 per cent., 
ground in linseed oil, turpentine and drier. 
This panel after three years' exposure was 
in good condition, although there was a 
slight checking. In test No. 2 the concrete 
was treated with a 20 per cent, solution of 
alum (aluminium sulphate), and a paint of 
the same composition as before. The 
results were almost identical with No. 1. 
In other tests the concrete was primed with 
a solution made by dissolving 10 parts of 
sodium oxalate in 100 parts of water. The 
second and third coats were linseed oil 
paints in red, brown, blue and green. The 
results are reported as being " very good." 
But even better results were obtained when 
the second and third coats were composed 
of zinc oxide and barytes ground in an oil 
(kind not given) having " a low saponifica- 
tion value." This paint dried very slowly, 
but the results are reported as " excellent,, 
extremely glossy waterproof surface pre- 
sented." 



Durability of Reinforced Concrete 



WHEN, recently, a reinforced concrete house, 
built fifty-eight years before at St. Denis, 
France, by Frangois Coignet, was examined 
and the work cut away to disclose the in- 
ternal condition, both steel and- concrete 
were found to be in an excellent state of 
preservation. It is a practical illustration 
of this kind, vouched for by a deputation 
from the (British) Concrete Institute, that 
carries conviction to the minds of those 
engineers who are unmoved by mere theories 
and by hundreds of laboratory experiments. 
That concrete has an effective life of hun- 
dreds of years under favourable conditions 
is an established fact ; further, those who 
have made a special study of reinforced con- 
crete know that in ordinary circumstances 
the steel will last as long as the concrete in 
which it is embedded. The concrete exer- 
cises a protective and preservative action 
on the steel ; and while he concrete gains 
in strength a hundredfold, the steel gains in 
durability a thousandfold. 

MECHANICAL DESTRUCTIVE 
INFLUENCES 

Blows and Shocks. Concrete is not an 
ideal material to employ in situations where 
heavy direct blows are likely to fall upon it. 
Obviously, its brittleness is its greatest 
defect, and it therefore follows that while 
it makes a durable and wellnigh everlasting 
floor for ordinary traffic, it is not suitable 
for the floors of workshops in which heavy 
iron implements are carelessly dropped 
about, or in which heavy trucks with flat- 
rim wheels are used, as rough treatment 
soon chips the surface. Truck wheels need 
to have curved treads or rubber tyres, so that 
there are no edges to dig into the concrete 
when turning a corner, even when a truck is 
tilted considerably. Hence, although con- 
crete makes an admirable floor foundation, 
it is frequently desirable to have the actual 
floor of wood, or to let in iron plates flush 
with the surface where strongly abrasive 
wear is likely to occur. As a floor material, 
too, concrete suffers with all stones, whether 
natural or artificial, in lacking in resilience, 



the " deadness " of the floor causing the 
people who stand or walk on it to become 
tired sooner than would be the case on a 
wooden floor. 

Reinforced concrete is being increasingly 
used for dock and harbour works, in which 
situations it is commonly subjected to great 
shocks. Mr. F. E. Wentworth-Shields 
(of the New Dock Works, Southampton) 
gives a valuable opinion with regard to its 
suitability for such applications. " If rein- 
forced concrete is to be used for marine 
work," he says, " it must be carefully 
fendered at all parts where it is likely to 
be struck. In most systems of reinforced 
concrete, the steel is placed within an inch 
or two of the surface ; and if the structure 
is not protected by timber fenders, this 
concrete skin is easily knocked away, leaving 
the steel naked and liable to rust. At the 
same time it is true that steel-concrete does 
not spall easily from blows, provided they 
do not fall directly on the concrete, and it 
will stand a wonderful amount of shocks, 
and bending due to shocks, if a wooden 
fender is interposed. The coaling-jetty at 
Southampton probably suffers as much from 
the blows of ships and barges as any steel- 
concrete structure yet made. It is pro- 
tected by fenders of American elm, and in 
spite of blows which have caused the whole 
jetty to sway, there is no sign as yet (two 
and a half years after completion) of the 
concrete spalling off, except at one or two 
places where the blow has fallen directly 
on the concrete." The same writer gives 
an interesting example of the ease with 
which reinforced concrete coast construc- 
tions can be repaired the ease with which 
repairs can be effected being important 
when broadly viewing the subject of dura- 
bility. He states that a steamer collided 
violently with the quay of this same jetty, 
breaking two piles and the beams which they 
supported. " On examination by divers it 
was found that the piles were broken down 
to a level of 12 ft. below low water. The 
work of repairing the quay, however, turned 
out to be fairly simple ; the broken concrete 



256 



DURABILITY OF REINFORCED CONCRETE 



257 



and steel rods were cut away by divers, who 
also erected new rods to take their place. 
These new rods were joined to the undamaged 
part of the old ones by steel tubes, and the 
stirrups or binding wire were then placed 
round the new rods at correct intervals. A 
water-tight casing of timber was then placed 
round the pile, which extended from the 
undamaged portion to above low water. 
From this casing the water was pumped out 
and the concrete was filled and rammed into 
the box in the ordinary way. Above low 
water the piles and beams were remoulded 
in just the same way as they had been origin- 
ally built." 

Earthquakes. These subject buildings 
to every form of stress and shock. Parts of 
a building designed to take simple com- 
pression are, in a moment, subjected to 
enormous bending strains, and floors are 
suddenly called upon to act as struts. Whilst 
no form of building can long resist severe 
earthquake attacks, reinforced concrete has 
given as good an account of itself as any 
other material, and a better account than 
many. There is, unfortunately, a diverg- 
ence of opinion on the subject, and, as might 
naturally be expected, the brick and terra 
cotta interests have made out a strong case 
for the superiority of their own materials. 
Captain Sewell, who personally studied on the 
spot the results of the San Francisco earth- 
quake shortly after the catastrophe, stated 
in an article in Concrete that at " Palo 
Alto, at the Leland Stanford University, 
there was a museum building which consisted 
of three wings : the central one of reinforced 
concrete, and the two side wings of brick- 
work, with reinforced concrete floor systems. 
This building was not far from the line of 
the fault which caused the earthquake, and 
it received a severe shaking. Externally, 
the reinforced concrete wing appeared to 
be absolutely uninjured, except that some 
statues were shaken down from the front 
parapet wall. The two brick wings were 
almost in a state of collapse. An examin- 
ation of the interior showed that some 
damage had been done in the reinforced 
concrete building, in the shape of a few 
cracks here and there ; but the sum total 
of structural damage was insignificant, and 
the writer is inclined to think that one 
thousand dollars would cover all the damage 
to this wing. The damage to the brick 
wings is at least from 50 per cent, to 75 per 
cent." 

17 



In 1908 there was a terrible earthquake 
disaster at Messina, in Southern Italy. 
Gf. Flament Hennebique, in a paper pre- 
sented to the French Society of Civil 
Engineers, described how certain Henne- 
bique structures at Messina had withstood 
the shocks. The more noteworthy instances 
are here mentioned. A roadway over the 
Portalegni stream remained intact in spite 
of the debris which accumulated upon it ; 
the floors of the Mandalari Hospital con- 
tributed largely to preventing the collapse 
of the building ; a 4,000 cub.-metre reservoir 
remained perfect ; the walls of the Messina 
museum collapsed and carried with them 
the floors, which remained unbroken; the 
church of the Madeleine collapsed, but the 
reinforced floors of the adjoining house 
remained intact ; the floor of the railway 
electric generating station, situated in the 
most severely shaken part of the town, 
did not suffer, although the upper parts of 
the building collapsed or were badly cracked ; 
the hospital attached to the medical school 
was destroyed, with the exception of the 
reinforced concrete parts, notably the stair- 
case, which remained standing in the ruins. 

Settlements. In a milder, slower, but 
actually surer way, settlements subject a 
building to the same kind of destructive 
stresses as those caused by earthquake. An 
appreciable settlement will inevitably lead to 
cracks, unless it is an even sinking over the 
whole area of the site. In a wooden or steel 
structure there is a slight accommodation to 
the consequences of a local settlement, on 
account of the elasticity of the material ; 
but there can be none in a monolithic 
building. The fact to be borne in mind is 
that the foundation should be of such a 
nature as to put the risk of settlement out 
of all question ; and the use of properly 
driven piles or of a reinforced concrete raft 
easily ensures that condition of affairs. 

Frost. This is a deteriorating influence 
which is at work upon most building 
materials. Few substances containing cal- 
careous compounds are free from its destruc- 
tive effects. Extremely porous substances 
are more liable than close-grained ones to 
be destroyed by a disintegration of their 
particles due to the freezing and consequent 
expansion of absorbed water. In the case 
of concrete, however, the effect of frost is 
only serious under certain limited conditions. 
The greatest danger likely to occur from frost 
is while the concrete is being gauged up and 



258 



REINFORCED CONCRETE 



before it has set, and, when necessary, 
special precautions must be taken at this 
stage. The effect of frost after the concrete 
has set is of little consequence, as reinforced 
concrete of good quality absorbs relatively 
little moisture. Where defective cement has 
been used, frost might constitute a very real 
danger. Valuable information with regard 
to the effects of frost upon concrete has been 
obtained by submitting briquettes of the 
material under varying conditions to different 
degrees of cold, and noting the tensile strength 
after certain periods have elapsed. Should 
a frost occur immediately after mixing, the 
strength of the concrete may be permanently 
affected ; but should it occur after twenty- 
four hours and before the end of the eighth 
day, and then be only slight, the setting 
will be retarded, but normal strength will be 
developed later. After the eighth day, the 
frost has no effect, but heavy frost of, say, 
15 is likely to injure freshly mixed concrete 
permanently. It is wise to refrain from 
mixing cement or concrete when the tempera- 
ture is below 29 F., which is the freezing 
point of the material. The detrimental 
effects of frost appear to be due not to 
chemical difference or changes, but purely 
to mechanical or physical ones, as before the 
cement has had time to exert its binding 
action upon the aggregate, the frost has 
already robbed it of its power to undergo 
the chemical changes characteristic of 
" setting." 

Much of the effect of frost depends on the 
amount of water present in the concrete, a 
very wet specimen not being nearly so seri- 
ously injured as a dry one. No satisfactory 
explanation of the action of frost has yet 
been made ; it appears to be due to the 
great thrust exerted when the water crystal- 
lises from the liquid state, but the nature 
of this " crystallising force " is, as yet, but 
imperfectly known. It appears to be much 
more powerful than the increase in volume 
which water undergoes on freezing and to 
which the disruptive action of frost is com- 
monly attributed. 

Fire. The ability to resist the destructive 
effects of fire is one of the most important 
factors in determining the utility of a build- 
ing material. The cement in concrete is 
ruined when its water of crystallisation has 
been driven off, this occurring at tempera- 
tures between 600 and 800 F. (316 and 
427 C.) ; but if the dehydrating action 
is slow, the material that has undergone 



it generally remains and insulates the rest 
of the work from the heat. The risk, how- 
ever, of extensive damage from this cause is 
not great, since serious dehydration requires 
an amount of time which would not be likely 
to have elapsed before the action of the heat 
would be quenched by water applied by 
the firemen. 

It is impossible to attempt even to out- 
line here the numerous experiments con- 
ducted under eminent authorities to test 
the fire-resisting properties of concrete ; 
little more can be done than to say that the 
result of the many accurate experiments 
has been such as to justify the conclusion 
that reinforced concrete is one of the very 
best fire-resisting building materials. Tests 
conducted by the British Fire Prevention 
Committee have shown that reinforced con- 
crete suffered no diminution of strength or 
decomposition to any appreciable extent 
by the heat, although the temperature was 
raised to as high a point as 2,000 F. (1,093 
C.). In 1906 Prof. Woolson subjected cubes 
of concrete to a temperature of 1,500 F. 
(816 C.) for two to five hours, and he found 
that, with the exception of specimens in 
which quartz gravel had been used in place 
of broken stone, the interior of the concrete 
did not attain the exterior temperature 
convincing proof of the low thermal con- 
ductivity of concrete, and pointing to the 
fact that when a concrete wall is heated on 
one side no serious rise of temperature is 
occasioned on the other side. 

Concrete blocks two months old were 
placed in boiler flues for twenty-one days 
and subjected to temperatures of 1,250 and 
250 F. (677 and 121 C.) night and day 
alternately ; there was practically no altera- 
tion in the condition of the concrete at the 
end of the period. 

During a fire that occurred recently in a 
large warehouse in Dresden, in spite of the 
fierceness of the flames, the firemen were able 
to keep the fire within the story in which 
it had originated, owing to the reinforced 
concrete construction. Subsequent examin- 
ation showed that the floor was practically 
unaffected, although it had been exposed to 
a temperature of nearly 2,010 F. (nearly 
1,100 C.), and only in one or two places had 
small parts of the concrete fallen off, laying 
bare the reinforcement, in spite of the fact 
that it was acted upon while at these high 
temperatures by the firemen's hose. 

A few years ago the United States 






DURABILITY OF REINFORCED CONCRETE 



259 



Geological Survey tested thirty panels of 
various building materials to determine the 
effects of fire and subsequent quenching 
with water. In most cases the materials were 
subjected to the direct application of gas 
furnace heat for two hours and then quenched 
with water. Figs. 470 to 475 are photo- 
graphs of six of the panels after quenching. 
The explanations under the figures should 
be read before attempting any comparisons 
between them. The report (by Richard L. 
Humphrey, an officer of the Survey) con- 
tains a number of significant statements, of 
which the more important are here quoted : 

" Much of the damage done to the building 
materials in this series of tests was occa- 
sioned by internal stresses, the gas flame 
heating one face of the test pieces much 
more rapidly than the other face. All the 
materials tested, including hydraulically 
pressed brick, cracked . more or less. The 
concrete cracked least, but the tests indicate 
the necessity for using metal reinforcement 
in concrete walls to distribute the effect of 
the expansion. 

" The average temperature attained by the 
faces of the panels ten minutes after the gas 
was lighted was about 324 C. (615-2 F.), 
and nearly half of the panels had been sub- 
jected to freezing weather just prior to the 
tests. The average temperature of the face 
of one panel of building blocks rose from 
<0 to 450 C. (32 to 842 F.) in the first ten 
minutes of firing, while that of another panel 
of the same material ranged from 22 to 
600 C. (71-6 to 1,112 F.) during the same 
interval. 

" A fact brought out most clearly by these 
tests is the low rate of heat-transmission in 
portland cement mortars and concretes. This 
is one of the desirable qualities in materials 
intended for fireproofing purposes. For 
marking the cement blocks, linen tags were 
fastened by wire nails to the interior walls 
at the time of moulding. Most of these tags 
remained in place during the test, and when 
the walls were dismantled the tags in every 
<;ase were found entirely undamaged. 

" There was a comparatively slight increase 
in the temperatures of the backs of the blocks 
during the test. The rise in temperature of 
the backs of the building blocks made of 
river-sand mortar varied from 25 to 40 C. 
(77 to 104 F.), while the rise in the aver- 
age temperatures of the faces of the cement 
blocks ranged from 650 to 900 C. (1,202 
1,652 F.). The backs of the mortar 



to 1,65 



blocks made of slag sand showed a rise of 
temperature of only 10 C. (50 F.), while 
the faces were heated up to 800 C. (1,472 
F.). The low rate of heat conductivity of 
the mortar blocks is shown by the slowness 
with which the temperature of the backs of 
the panels increased in comparison with 
that of the faces. 

" The damage done to the faces of the 
mortar and concrete panels would probably 
be caused at a temperature about half that 
of the maximum temperatures reached, owing 
to the water of crystallisation being driven 
from a layer of the mortar. 

" The backs of the brick panels also showed 
a small increase in temperature, but the 
natural building stones and the tiles proved 
poorer non-conductors of heat. The temper- 
ature of the back of a panel composed of 
plastered tiles increased to 128 C. (262-4 F.). 

" It was difficult to determine whether the 
limestone, granite, gravel, or cinder concrete 
sustained the least damage. The faces of all 
the panels were more or less pitted by the 
fire and washed away by the stream of water. 
The test was unfair to the cinder concrete, as 
the cinder was very poor, containing a large 
percentage of unburned coal ; however, the 
sample selected was the best of the six or 
eight investigated when purchasing. During 
the fire test the coal ignited and left the sur- 
face of the concrete very rough and badly 
pitted. The limestone aggregate in the face 
calcined, and the granite aggregate split and 
broke away from the surface mortar. The 
granite concrete probably behaved the best. 
The damage in no case extended deeply, 
probably not more than 1| in. The evidence 
shows that even at this depth the tempera- 
ture was comparatively low. The high 
stresses produced in the panels by the rapid 
rise of temperature of the faces while the 
backs remained cool caused cracks. On 
taking down the panels the blocks of con- 
crete were found to be cracked vertically for 
some distance back from the face. 

" The serious damage to the various 
natural building stones precludes any com- 
parison among them. 

" The concretes tested were pieces of 
broken beams that had been tested in the 
Government's structural materials testing 
laboratories. The pieces were 8 in. by 11 in. 
in cross section and of various lengths. They 
were fired with the 11-in. face exposed. In 
making the concretes, four distinct types of 
aggregate had been used, limestone, granite, 




Fig. 470. Single-air-space Mortar Blocks (Sand 
and Portland Cement) Laid in Fireclay, 
after Firing and Quenching 

These 63-day-old blocks were fired for 121 mins. (max. 
temp. 700" 800 F.) and quenched for 5 mins. with 
water at 51 F. A strip to the left of the panel was 
not quenched. After cooling all the blocks were 
found cracked across the web. Surfaces of the blocks 
could be broken into small pieces by gentle tapping. 



Fig. 471. Double-air-space Mortar Blocks, 
after Firing and Quenching 

Fired for 120 mins. (max. temp, about 650" F.), and, after- 
lapse of 19 mins., quenched fur o mins. Alter tiring^ 
vertical cracks showed on each side of the mitre 
webs. Richer wet blocks withstood better than the 
lean damp blocks. In some cases, end webs split 
away from and parallel to faces of blocks, whereas 
the middle web remained intact and the face split. 
away from it at each side. 





Fig. 472. Terra-cotta Three-air-space Partition 
Tiles Laid in Cement Mortar, after Firing 
and Quenching 

Plastered with ordinary lime plaster } in. thick, and 
finished witli a thin skim of plaster-of-paris. Fired 
for 121 mins. (max. temp, about 900 F.) and quenched 
for 5 mins. with water. Soon after starting firing-, 
the plaster came away, and cracks 'developed. In 
removing the panel from the furnace, the door stuck 
half-way, and the panel was badly shaken in en- 
deavouring to prise the door open. Thirty-seven 
mins. elapsed before the water was applied, the water 
washing away about 65 per cent, of the tiles that 
were exposed directly to the stream. 



Fig. 473. Various Building Stones Laid in 
Cement Mortar, after Firing and Quenching 

" Bedford" indicated in the illustration refers to Bedford 
limestone. Fired for 120 mins. (max. temp, about 
850 F.)and quenched for 5 mins. with water at 57 F. 
During firing, the sandstone spalled ; the limestone 
spalled and developed fine irregular cracks ; nearly 
all the granite blocks showed small vertical cracks ; 
the Bedford limestone showed small vertical cracks 
about 8 in. apart ; the marble showed signs of calcin- 
ation ; and the wall bulged 1,-^ in., this being reduced- 
after quenching to \ in. Alter cooling, the granite,, 
limestone and Bedford stone were cracked and had 
the faces badly washed off. 



260 



DURABILITY OF REINFORCED CONCRETE 



261 




gravel, and cinder. The aggregate was sized 
to pass a 1-in. screen and be retained on a 
^-in. screen. The concretes had been mixed 
in proportions of 1 of cement, 2 of Merrimac 
river sand, and 4 of aggregate, and were of 
.edium consistency." 

F. E. Wentworth-Shields, in the paper 
Iready quoted from, says it appears " that 
for fire-resisting purposes it is best that the 
steel reinforcement should consist of many 
bars of small section in preference to few 
Tsars of large section, as the first arrange- 
ment forms a close network which prevents 



instance, at a tobacco warehouse built at 
Bristol on the Coignet system, the walls 
consist of a framework of steel-concrete 
with brick panels. The brickwork can be 
arranged, as at Bristol, to conceal the con- 
crete entirely ; but the collapse of any part 
of the brickwork would not endanger the 
floors or the concrete framework." 

It will be noted that cinder concrete is 
recommended for the outer face a recom- 
mendation which the results of the United 
States Geological Survey tests do not seem 
to endorse. In the earlier chapter on con- 




Fig. 474. Concrete Blocks Laid in Fireclay, 

I after Firing and Quenching 
Concrete was of four kinds namely, limestone, cinder, 
granite, and gravel, all being mixed with portland 
cement and river sand. Fired for 123 mins. (max. 
temp, slightly exceeding 800 F.) and quenched for 5 
mins. with water at 52 F. During firing small pieces 
fell from the faces, and the limestone, gravel, and 
cinder concretes were pitted, the latter two suffering 
the most. 
. 

pieces of concrete from dropping out and 
so reducing the strength of the building. 
It is also important that the concrete aggre- 
gate be crushed small. The ideal composi- 
tion for the aggregate seems to be a strong 
flint or granite for the hearting, with an 
outside sldn of cinder concrete to act as a 
good non-conductor. It is essential, too, 
that in a building the walls, floors, and 
posts should be well tied into one another. 
There is a tendency, nowadays, in design- 
ing steel-concrete buildings to reject that 
material for the walls on the score of ex- 
pense, using it only for floors and columns. 
Sometimes a compromise is made ; for 



Fig. 475. Granite Blocks Laid in Cement 
Mortar, after Firing and Quenching 

Firing for 12<H mins. (max. temp. 900 F.) and quenched 
for 5 mins. with water at 57 F. During firing, panel 
bulged 3 in. and some of the joints and blocks 
cracked. Three blocks showed signs of spalling. 
During quenching, spalling and cracking became 
general. After cooling, the surfaces of the stones 
that remained intact could be easily rubbed off with 
the fingers : a gentle tapping broke the blocks into 
fragments. 

crete a note is made of the difference of 
opinion regarding this material ; but not 
only do opinions differ, even experiments 
appear to give contradictory results. For 
instance, fire tests of cinder concrete walls 
were conducted at Columbia University over 
a period of at least two years. The walls of 
the buildings used in subjecting floor con- 
structions to fire tests were given five fire 
tests of four hours each, during which the 
temperature was raised to about 1,700 F. 
(nearly 930 C.). At the end of each test a 
stream of water at 60 Ib. nozzle pressure was 
played to and fro over the ceiling for ten 
minutes, while the ceiling wall was red hot. 



262 



REINFORCED CONCRETE 



After this severe treatment, the walls, ac- 
cording to Prof. Ira H. Woolson, were prac- 
tically as good as they were when new, 
except that the rear wall was scored by the 
water to a depth of from \ in. to 1 in. for 
2 ft. or 3 ft. near the top. Prof. Woolson 
said he believed the structure to be good, 
with occasional plastering, for an indefinite 
number of tests. In his opinion, the per- 
centage of coal and the amount of fire 
material in cinder concrete have, within 
reasonable limits, very little if any effect on 
its fire-resisting qualities. He believes that 
sifting and washing the cinders would be a 
useless expense, and injurious to them as 
an aggregate for concrete. The pieces of 
coal which were close to the surface in the 
walls were burned to an ash, which re- 
mained in place and acted as a non-conductor 
of heat. Plenty of particles of pure coal 
could be found in the walls less than 2 in. 
beneath the surface. 

: It is well known that the water used in 
extinguishing a small fire generally does 
more damage than the fire itself. Where 
there is much wood and plaster construction, 
the water swells the wocd and brings down 
the suddenly quenched plaster ; it pours 
through the ceilings and does infinite harm 
in the floors below ; on the subsequent 
shrinkage of the wood there is further 
trouble, and more plaster is cracked. On 
the other hand, the quenching of a localised 
fire does not appreciably affect the rein- 
forced concrete, and as the floor is water- 
tight (practically considered) there is no 
harm done to the room below. 

CHEMICAL DESTRUCTIVE INFLUENCES 

In determining the life of building mate- 
rials and their powers to withstand the many 
influences which lead to decay, the test of 
time is by far the most accurate and satis- 
factory ; but the careful study of the 
chemical constituents of a building material 
may sometimes lead to a means of minimis- 
ing the effects of destructive influences. It 
will be shown in this section that while 
waters or atmospheres impregnated with 
chlorides or sulphates and acid-reacting sub- 
stances have generally but little effect upon 
concrete, there are 'many cases where a 
special knowledge of such actions may be 
desirable, and, in fact, essential, as in the 
construction of chemical factories, public 
conveniences, stables, etc., where reacting 
gases are likely to permeate the material of 



the building. In such circumstances, ample 
knowledge of the chemical action of certain 
gases and liquids would be useful in arriving 
at a means of preventing decay. 

Atmosphere. An average sample of 
atmospheric air has the following com- 
position per one hundred volumes : Oxygen, 
20-61 ; nitrogen, 77-95 ; carbonic acid gas, 
0-Oi ; aqueous vapour, 140 ; nitric acid and 
ammonia, traces ; carburetted hydrogen and 
sulphuretted hydrogen, traces in towns. 

The oxygen and nitrogen are without 
action upon concrete, but the former, in the 
presence of moisture, oxidises iron and steel, 
so forming " rust." 

Nitrogen is one of the most inert sub- 
stances known, and may be entirely disre- 
garded so far as concrete is concerned. 

Carbonic acid may slightly influence the 
condition of concrete. It is produced by 
the solution of a gas, carbon dioxide (CO.,), 
in water. The cement, which acts as a 
matrix in the gauging of the concrete, is 
composed chiefly of calcium alumino-silicates. 
There is also a certain amount of carbonate 
of lime (strictly, calcium carbonate), which 
is slightly soluble in water charged with car- 
bon dioxide (3 parts by weight in 1,000), the 
solution containing the acid calcium carbon- 
ate, H 2 Ca(C0 3 ) 2 . The proportion of car- 
bon dioxide in the air varies considerably 
with the locality, and in the vicinity of 
large factories appreciable quantities of it 
are brought down in solution in rain. In 
some cases this would be a factor in acceler- 
ating the decay of concrete in course of 
time ; but, generally speaking, under ordinary 
atmospheric conditions it may be entirely 
disregarded. 

Nitric acid, of which the atmosphere con- 
tains traces, is a fuming, powerful liquid 
(HN0 3 ), which, when brought into contact 
with alkaline and basic substances causes 
chemical action to take place, resulting in 
the decomposition of the substances acted 
upon ; but the proportion of this acid pre- 
sent in the atmosphere is too small for it to 
have any appreciable effect upon concrete. 

The other substances mentioned as con- 
stituents of the atmosphere are in far too 
small proportions to be of consequence in 
destroying concrete ; and, speaking gener- 
ally, therefore, the atmosphere has no 
chemically destructive influence upon con- 
crete, a statement substantiated by the 
stability of concrete after hundreds of years 
of exposure. Of course, where concrete is 



DURABILITY OF REINFORCED CONCRETE 



263 



used in buildings in which it may be exposed 
to any gaseous fumes, as in the manufacture 
of chemicals, the action of these gases upon 
the concrete requires special consideration. 

Whether or not the moisture in the air 
affects the steel reinforcement will be con- 
sidered under the next sub-heading. 

Water. Many authorities have feared 
that water must inevitably corrode the rein- 
forcement and bring about failure, the danger 
being all the more serious because the steel 
is completely hidden from view, and the 
course of the corrosion cannot be watched. 
The fear seems a reasonable one until certain 
proved facts relating to the chemistry and 
physical conditions of concrete and steel 
have been stated. In the first case, a thin 
skin or shell of concrete is not waterproof 
when new, although it may become so in 
the course of a couple of years or so. Natur- 
ally, the better the concrete the less will be 
its porosity ; but even the best concrete will 
absorb a very small amount of water, which 
naturally may find its way to the steel. 
There are now two facts to remember : 

(1) Iron or steel cannot rust ivhen damp 
except free oxygen be present. The concrete 
used in reinforced work should be of high 
quality and therefore of minimum porosity. 
Although it may permit the passage of an 
extremely slight amount of water, air at 
atmospheric pressure could not penetrate it, 
and rust cannot therefore form. But per- 
haps it will be said that the microscopic 
particles of water may carry entrained air 
with them. While this is scarcely likely 
probably quite impossible it could still be 
granted, and absolute reliance placed on the 
next point. 

(2) Steel cannot rust when embedded in a 
strongly alkaline substance. In the setting 
of portland cement a relatively large 
quantity of caustic lime (lime hydrate) is 
produced, and the concrete becomes satur- 
ated with it. This is strongly alkaline, and 
by decreasing the number of hydrogen ions 
present makes any corrosion of the steel 
impossible. 

On the other hand, if the concrete were 
acid instead of alkaline, the number of 
hydrogen ions would be increased, and, 
assuming the presence of oxygen, the steel 
would be corroded. Iron rust, it will be 
remembered, is not simply an cxide ; it is 
a hydrated oxide, one formula being 2FeO ;5 
+ 3H 2 ; this does not express the whole 
of the truth, since iron rust contains, 



besides the oxide, carbonic acid and am- 
monia. It is known that the composition 
of rust varies with surrounding conditions. 

Of supreme importance is the necessity 
of using the best concrete obtainable. 
" Rotten," porous concrete, permitting the 
free passage of water, would not maintain 
the steel bright and free from corrosion, 
since the caustic lime is soluble in water, 
and in time would all be dissolved away. 

Not only does concrete prevent the corro- 
sion of the reinforcement, but it removes 
any rust that may have been present on 
the steel. For this reason no concern is 
felt over the use of rusty rods and bars, but 
that is a mistake. Thick rust reduces the 
sectional area of the steel and lessens the 
adhesion of the concrete, although it becomes 
completely absorbed. 

It is claimed that experiments have 
proved that steel reinforcements to concrete 
are found to corrode rapidly when an electric 
current is passed through them. Such 
corrosion can take place only when the 
interior of the concrete is damp (that is, 
when the concrete is inferior), and need not 
be feared when material of the proper 
quality is employed. The rods, in the 
presence of moisture, would act as poles in 
a battery, and hydrogen would congregate 
round some and oxygen round others a 
condition conducive to rust. 

From certain authoritative experiments, 
it appears that the only risk of reinforce- 
ments rusting is likely to occur after the 
material has been stressed to 35,000 Ib, per 
square inch, a condition of affairs which 
ordinary designing renders absolutely impos- 
sible in normal circumstances, since the 
tensile strength of structural steel itself is 
regarded as about 60,000 Ib. per square inch ; 
the elastic limit is taken at about half that, 
and the factor of safety is 4 or 5. It is 
obvious that for rusting to be made possible 
such a stress must be applied as will give 
the steel a permanent set, and, by reduc- 
ing its diameter, destroy or, at any rate, 
seriously impair the cohesion between the 
steel and concrete. Such a stress cannot 
occur unless the designing be absurdly and 
obviously at fault. 

It will be of advantage in this place to give 
the conclusions arrived at by Ernest R. 
Matthews, A.M.Inst.C.E., after a number 
of tests, and presented by him in a paper 
entitled " The Corrosion of Steel Reinforce- 
ment in Concrete," read before the Society 



264 



REINFORCED CONCRETE 



of Engineers. Those conclusions are as 
follow : (1) Rusty steel embedded in con- 
crete will in a very short time become 
bright, regardless of whether the concrete is 
in water or air. This point has, in Mr. 
Matthews's opinion, been conclusively proved 
by his experiments. (2) The application of 
cement grout to steel is an effectual safe- 
guard against corrosion, but the greatest 
care should be taken in the grouting process 
to see that every portion of the steel is well 
coated ; and before the steel is embedded 
in the concrete the cement grout should be 
allowed to dry on the steel. (3) If the aggre- 
gate used for the concrete is not porous and 
the concrete is well mixed, the reinforcement 
being well embedded, no cement coating is 
needed. This is proved by experiment. 
(Seeing that the application of a coat of 
cement grout is such an inexpensive proced- 
ure, Mr. Matthews makes it a rule in carrying 
out work of this kind to have all reinforce- 
ments coated in this manner.) (4) No 
porous materials, such as coke breeze or slag, 
should be used in connection with reinforced 
concrete work if such concrete is intended 
to be under water or exposed to the air. (5) 
Linseed oil or turpentine, or probably any 
other coating except cement or lime, applied 
to steel before its insertion in concrete, 
facilitates rather than prevents the rusting 
of the metal. (6) It is of great importance 
to ensure that the reinforcing steel is well 
embedded in the concrete, so that every 
portion is covered with cement. (7) The best 
results were obtained with aggregate con- 
sisting chiefly of broken stone or brickbats. 
Gravel would no doubt answer equally well. 
Sea Water. Whilst pure water is with- 
out chemical effect upon concrete, sea water, 
acting both chemically and mechanically, 
exercises an influence that must not be dis- 
regarded. To form some idea as to the 
probability of sea water affecting concrete, 
it is desirable to know its constituent parts. 
An average sample contains per 1,000 parts 
the following compounds or salts : 

Sodium chloride 27-059 

Magnesium chloride . . . . 3-666 

Magnesium sulphate . . . . 2-296 

Calcium sulphate . . . . . . 1-406 

Potassium chloride . . . . . . -766 

Calcium carbonate . . . . . . -033 

Magnesium bromide . . . . -029 

Total salts 35-255 

Water . 964-745 



In the foregoing list the magnesium and 
calcium chlorides and sulphates are the only 
compounds likely to have appreciable effect 
on the chemical nature of concrete ; such 
action as they possess is due to the ions or 
acid radicles which they contain, and, 
although it is slight, the total quantity 
of these salts is so large that the effect 
on the concrete is much greater than the 
small proportion of them in the sea water 
suggests. 

As regards the other salts present in sea 
water, the chief of which is common salt or 
sodium chloride, their action appears to be 
of but little consequence. In cases, how- 
ever, where sea sand has been used in the 
gauging, the concrete takes longer to set, 
and it presents a damp surface for some 
time, due to the magnesium chloride and 
other hygroscopic salts in the sand, derived 
from the sea water, absorbing moisture from 
the atmosphere and so remaining damp. 

A very practical contribution to the dis- 
cussion of sea-resisting concrete is a Danish 
paper read before the Copenhagen meeting 
of the International Committee on Rein- 
forced Concrete, by A. Poulsen, who gives 
the conclusions drawn from two series of 
experiments ; the following is according to 
Dr. Desch's summary of the paper : (1) 
Good portland cements are very resistant to 
the action of sea water. No marked differ- 
ence in the behaviour of cements slightly 
different in composition has been found, 
except that a high proportion of aluminates 
tends to cause disintegration. (2) In a 
dense mortar, the chemical action is con- 
fined to an outer layer of small depth, further 
action being checked by the slowness of 
diffusion. A porous mortar, by admitting 
salt water to the interior, is apt to crack by 
expansion owing to chemical change. (3) 
The main agency in the destruction of 
mortar and concrete in marine embankments, 
harbour works, groynes, etc., is not chemical 
action, but the alternations of saturation, 
drying in the sun, freezing, etc., due to the 
alternate exposure and covering by the rise 
and fall of the tide. Destruction takes place 
sometimes by cracking, sometimes by scal- 
ing, the latter effect being produced especi- 
ally by frost. (4) The denser the mortar the 
better (1 cement to 3 sand is too poor for 
marine work). An admixture of fine sand 
with the ordinary sand increases the close- 
ness of the mixture, but a well-graded ag- 
gregate is the most advantageous. (5) The 



DURABILITY OF REINFORCED CONCRETE 



265 



addition of finely-ground silica or trass to 
the cement before mixing is often advan- 
tageous in the case of the more porous 
mortars which are rich in cement. It is 
very doubtful whether anything is gained 
by adding trass to the denser mortars, 
which are rich in cement. (6) Hydraulic 
lime mixed with trass, etc., is of Borne 
value where a cheap material is required, 
in the mild climate and absence of tide of 
the Mediterranean, but it is incapable of 
withstanding the conditions of coast work 
in northern latitudes. (7) The destructive 
action of the sea being mainly physical and 
mechanical, and not chemical, tests by mere 
immersion in still sea water are of very little 
value in determining the behaviour of con- 
crete in marine engineering works. A mix- 
ture which disintegrates under this test is 
certainly useless, but a mixture which passes 
the test may disintegrate under the more 
stringent conditions of practical use. (8) As 
long a period as is practical should be allowed 
for the hardening of concrete blocks before 
placing in the sea. The German custom of 
leaving them for one year in moist sand be- 
fore setting in place is impracticable in many 
places, but should be approached as nearly 
as possible. (9) The behaviour of test- 
specimens for the first twelve months is very 
irregular, and definite conclusions can only 
be drawn from the results of long-period 
tests. 

Acids. Hydrochloric and sulphuric acids, 
while scarcely affecting siliceous materials 
such as sand and concrete aggregates, 
decompose the cement by dissolving out 
the soluble substances such as calcium 
carbonate, and they readily attack the 
aluminium compounds in the cement. Sul- 
phates have a less powerful action than 
have chlorides, and they form products 
which are largely insoluble, and which, 
therefore, serve as protective coatings. 
Fortunately, as concrete consists largely of 
sand and aggregate, which are scarcely 
affected by ordinary acids, it offers a 
natural resistance to the common acids 
which would be liable to form by the action 
of chlorides or sulphates. 

The action of hydrofluoric acid is one 
worth consideration, as, although a rare acid, 
its action is certainly of interest in special 
cases. The most characteristic property of 
this acid is its readiness to attack silica 
either in the combined state as silicates or 
in the free state as quartz sand. In fact, 



it is the only acid which will dissolve this 
substance. The acid may be formed in a 
variety of ways, such as by the interaction 
of the minerals fluorspar or cryolite and 
sulphuric acid, which produces or at 
least releases the powerful gaseous hydro- 
fluoric acid, which is known to attack 
almost anything with which it comes into 
contact. 

As has already been explained, small 
quantities of nitric acid are brought down 
by the rain, but larger quantities of nitrates 
are found in soils, where they are produced 
by the decomposition of vegetation and 
other nitrogenous matters ; in cases where 
the concrete comes into actual contact with 
substances of this nature there is no doubt 
that in course of time it will be affected by 
the chemical action of the nitrates. 

Many organic acids are without action 
upon concrete, but in the presence of sodium 
chloride (common salt) they are capable of 
dissolving the cementitious portion of it to 
a slight extent. Hence the action of saline 
solutions, such as sea water, on the cement 
present in concrete, may in some instances 
prove serious. 

Alkalies. Such alkalies as strong potas- 
sium hydrate (caustic potash) and sodium 
carbonate (soda) are capable of dissolving 
silica, a substance that is not acted upon 
by any acids except hydrofluoric. 

Urine. The action of this substance is of 
importance in view of the employment of 
concrete for stables, public conveniences, 
etc. Possibly, should concrete prove a 
durable material for such structures, a suit- 
able surface finish will be evolved, so 
obviating the employment of the relatively 
costly tiling now in common employment. 
Urine contains water, urea, uric acid 
and certain organic and inorganic salts. 
Of these constituents the one most likely 
to influence substances of the nature of 
concrete is urea. By the action of 
moisture, urea (NH 2 ) 2 CO, is turned into 
ammonium carbonate (NH 4 ) 2 C0 3 , which 
attacks silicates and substances of a 
siliceous nature, and hence tends to 
cause the decay of concrete in course of 
time. Uric acid is a weak acid, which is 
readily changed into urea by exposure to the 
atmosphere. The action of urine upon con- 
crete would certainly be a slow one, but it 
calls for consideration where the durability 
of concrete in the construction of stables, 
etc., is concerned. 



Waterproofing Concrete 



THE permeability of concrete has proved a 
matter of great concern to many engineers 
and users of this material, especially in the 
construction of water tanks, sewers, aque- 
ducts, gas-holders, etc., and numerous 
attempts have been made to render it im- 
permeable to water, oil, and other liquids. 
The methods used to render concrete 
waterproof are of two main classes (a) those 
in which a material of an impervious nature, 
or one which will combine with an ingredient 
in the concrete to form an impervious 
material, is applied to the surface of the 
finished concrete, and (6) those in which 
substances are mixed with the concrete so 
that the whole mass becomes impermeable. 
The latter method is, in many ways, the 
most effective, particularly if the surface of 
the concrete is likely to be damaged, as it 
gives the whole mass the maximum im- 
permeability ; but it is also more costly, and 
cannot be applied to the finished work. 

SUPERFICIAL WATERPROOFING 

Superficial waterproofing may be accom- 
plished in a variety of ways, of which the 
chief are (a) coating the surface with an 
impervious material, such as cement grout- 
ing, dense mortar, tar, asphalt, or paraffin 
wax dissolved in light petroleum ; or one 
which becomes impervious on drying as 
sodium silicate (water glass) and some casein 
paints followed by formaldehyde or a solu- 
tion of alum followed by a solution of soap, 
or soda lye, these alternate coatings being 
repeated as often as is considered necessary ; 
and (6) applying to the surface a substance 
which will enter the pores in the concrete and 
will form with the cement an insoluble and 
impermeable compound. Sodium oxalate, 
which forms the insoluble oxalate of lime in 
the pores of the concrete, is typical of this 
class of waterproofing material, as is linoleic 
acid, which forms an insoluble lime soap ; 
but the most largely used substances of this 
class are soluble silico fluorides, under the 
commercial term of fluates. These fluates 
are chiefly composed of aluminium, zinc or 
magnesium silico fluorides, and when applied 



to concrete they combine with the lime set 
free as the cement sets and, by forming double 
silico fluorides, they not only render the 
material waterproof, but also increase the 
hardness and durability of the surface to 
which they are applied. M. Merkuloff has 
shown that cubes composed of 1 part of 
portland cement to 3 parts of sand when 
immersed in a dilute fluate solution for 
several hours increased 50 per cent, in com- 
pressive strength, and that their resistance 
to the action of frost was similarly increased. 
The surface of the cubes was found to be ten 
times as resistant to abrasion as that of the 
untreated cubes. 

MASS WATERPROOFING 

By far the most effective means of render- 
ing a mass of concrete waterproof is so to 
arrange the aggregate as to obtain a material 
practically devoid of pore spaces and with 
the closest possible texture. For poor 
concretes this may necessitate the addition 
of an exceedingly fine, insoluble powder, 
such as slaked lime or china clay. A careful 
grading of the aggregate and the sand used 
will produce a fairly close concrete, and 
then a pore filler is quite unnecessary, as has 
been proved over and over again. When pore 
fillers are employed, they should be added; 
to the cement or concrete in a dry state, 
and be well mixed before water is added. 
Plastic clay is not a suitable pore-filler : 
it is too adhesive; but powdered china 
clay is suitable. Waxes, greases and oil 
are similarly inferior to china clay and to 
slaked lime. The only objection to slaked 
lime is its tendency to produce a whitish 
efflorescence on the surface of the concrete' ; 
china clay does not do this, but its slight 
plasticity is an almost equal drawback. 
If sand or calcined clay should be obtained 
in a state of fineness equal to that of china 
clay it would be almost perfect as a pore- 
filler, but, unfortunately, no grinding plant 
will produce such a fine product on a com- 
mercial scale at a price at which it can be 
used for this purpose. 

As showing the differences in" permeability 



266 



WATERPROOFING CONCRETE 



267 



due to the use of equal weights of various 
substances in the same aggregate, the follow- 
ing table (due to Concrete) is interesting. 
The figures in the third column represent the 
number of cc. of water which penetrated 
concrete slabs 10 in. square and 3 in. thick, 
subjected to a water pressure on the face of 
50 Ib. per square inch during twenty-four 
hours. The slabs were taken from the 
moulds twenty-four hours after gauging, 
and were kept in cold water for twenty days 
before use. 



Series 



3 per cent, waterproofing 
material added to cement 



A 
B 

C 
D 
E 



G 

H 
I 
J 
K 

L 
M 



Nil 

Mixture of slaked lime 

and lime soap . 
Kesin 
China clay 
Equal parts resin and 

china clay 
Paraffin wax . 
90 parts slaked lime, 10 

parts paraffin wax 
Slaked lime 
Burned gault clay . 
Alumina . 
90 parts china clay, 10 

parts wax 
Equal parts china clay 

and alumina 
5 per cent, china clay 



Permea- 
bility 



1240 

1290 
760 
865 

740 

1005 

555 

450 

2370 

320 

280 

372 

450 



That the quantity of water used in gauging 
concrete has a large influence on the im- 
permeability of the mass has been well 
established, but Cloyd M. Chapman has 
shown that the least absorption occurs when 
the concrete contains 13 to 15 per cent, of 
water, as shown in Fig. 476, which sum- 
marises the results of tests on 230 blocks. 

The best method of obtaining a water- 
proof material thus appears to consist in 
using a carefully selected aggregate, together 
with the proper proportions of fine material 
and cement. 

If the proportions of water, cement, sand, 
and aggregate are correct, and the aggregate 
is properly graded, there should be but little 
difficulty in obtaining a mass that is suffi- 
ciently waterproof for all practical purposes. 
The addition of a pore-filler to such a mass 
is unnecessary. 



Water-repellent substances are also 
used in admixture with concrete, the idea 
being that a substance which is not merely 
insoluble, but actually repellent to water, 
must be more advantageous than such a 
substance as clay, silica, or alumina. Most 
of these water-repellent materials are of a 
fatty or oleaginous nature, oils, soaps, and 
the corresponding fatty acids, such as stearic 
and linoleic acid, being chiefly used. The use 
of blood, milk, and lard is extremely ancient, 
being recommended by Vitruvious. At the 
present time, mixtures of lime and tallow or 
of soap and alum are preferred. 



s 10 



| 9 "0 II 12 13 14 15 16 17 IB 19 20 
fercentagf o] tt/atei uscO in mning Concrete 

Fig. 467. Graph Showing Relative Water 
Absorption of Concrete 

These materials are usually supplied in the 
form of a powder which is mixed with the 
cement before gauging, but some workers 
prefer to keep solutions of the ingredients 
forming the water repellent, and to mix 
these solutions separately with the concrete. 
Thus, if a solution of alum is first added to 
the gauged material and then a solution of 
soda, there will be formed in the pores of 
the material a precipitate of aluminium 
hydroxide. Unfortunately, experiments have 
shown that this precipitate is very irregu- 
larly distributed, and that it does not 
increase the impermeability as much as the 
corresponding amount of finely divided 
alumina added to the concrete previous to 
the latter being mixed. 

Emulsions, consisting of oil in the form 
of "minute globules suspended in water, are 
also used to render concrete impervious to 
water. Their use is based on ignorance, as 
oils added to the mixing batch are more 
convenient, and the emulsion formed during 
the mixing process is better distributed 
through the concrete than when an emulsion 
is added in the first place. 

Oils mixed with concrete, in the propor- 
tion of 10 to 15 per cent, of the weight of 



268 



REINFORCED CONCRETE 



the cement used, are used to render con- 
crete 1 waterproof. The oils used for this 
purpose must be of mineral origin, as animal 
and vegetable oils are liable to form acids 
which disintegrate the concrete. The most 
suitable for the purpose are mineral oils 
of various kinds, and range from heavy, 
black bituminous oils to light, non-volatile 
petroleum oils ; white or colourless oils are 
used where the colour of the concrete is 
important. 

It is unnecessary to add oil to good 
concrete ; the addition will render it less 
waterproof and will reduce the strength. 

A great disadvantage in the use of oil is 
the delay which occurs both in the initial 
and final setting and hardening and the 
loss of strength and toughness which 
ensues. 

The chief objection to adding substances 
of a water-repellent character to concrete or 
to the cement in the form of a dry powder is 
that the natural tendency of such materials 
is to leave the cement when water is added 
and to collect in small masses, whereby a 
material of irregular composition is pro- 
duced. The same objection holds good in 
the case of oleaginous and other substances 
which are mixed with water before use ; 
the water merely carries the particles hetero- 
geneously into the concrete mass, and does 
not effect a uniform distribution. Moreover, 
the use of materials which possess a strong 
water-repellent action undoubtedly tends to 
reduce the strength of the concrete with 
which they are mixed. 

Extensive experiments with a large variety 
of substances (each of which was mixed with 
the cement in the proportion of 3 of substance 
to 100 of cement before use) show that water- 
repellent substances, such as oils, greases, 
soaps, resins, etc., have no true water- 
proofing qualities, and the slight benefit 
they confer is more than counterbalanced 
by the extent to which they weaken the 
concrete. 

MAKING A NON-POROUS CONCRETE 

The impermeability of concrete when 
obtained is due primarily to the absence of 
pores or voids in the material, and this 
can only be secured efficiently by a careful 
selection of material of suitable grades. For 
example, if sufficient cement is used and the 
mixing is thoroughly performed, all the pores 
into which water can penetrate will be filled. 
When, however, insufficient cement is used, 



a non-hydraulic material of equal fineness 
may be added to fill the pores. The amount 
of this material which, as already indicated, 
may be of any convenient composition 
providing it does not attack the other 
constituents of the concrete must be ascer- 
tained by experiment. The only satisfactory 
method consists in making a number of 
blocks with aggregates of various grades and 
to test the permeability or porosity of the 
blocks. The most satisfactory mixture is 
then made the basis of a further series of 
tests in which different percentages of pore- 
fillers are used. In this way, with care, it 
is usually possible to produce a perfectly 
waterproof concrete with little or no pore- 
filler in it. 

Concretes that are easily penetrated by 
water and other neutral liquids will usually 
be found, on examination, to be composed 
of badly graded aggregates or to contain a 
considerable proportion of very coarse 
material. The larger the blocks the larger 
may be the size of the coarsest pieces of 
aggregate, but blocks of small size should 
not contain pieces of material which will 
not pass through a J-in. ring. The great 
difficulty experienced by some engineers in 
obtaining sufficiently waterproof concrete 
may usually be overcome by the use of 
material which will not pass through a 
-in. or f-in. ring, and by screening out of 
the still coarser aggregate (if very large 
blocks are used) all matter in it which is less 
than in. in diameter. The use of too coarse 
an aggregate and of an aggregate containing 
too large a proportion of fine material are 
both equally detrimental to impermeability. 

It is important to note that a concrets 
should not be judged too hastily as regarde 
its waterproof qualities. Until it is set all 
cement is porous, and many instances are 
known where the " weeping " of water-tanks 
ceased after the cement in the concrete 
had become properly hardened. Conversely, 
concrete which is subjected to the action 
of impure water (such as sea water or 
spring water highly charged with carbon 
dioxide gas) may be rendered permanently 
porous, whereas if it had been allowed to 
harden properly before being used it would 
have proved quite impervious. 

It is frequently found that concrete that 
is insufficiently waterproof has been care- 
lessly made or indifferently mixed. In such 
cases any supplementary waterproofing that 
may be necessary must be largely of a super- 




WATERPROOFING CONCRETE 



269 



ficial character, and will consist of the 
application of fluates, water glass, or some 
penetrable paint. The external application 
of ordinary paint is useless until the free 
lime in the concrete has been neutralised 
by one or more preliminary coats of a suit- 
able acid. A coating of tar or asphalt is 
only available when the pressure of the 
water on it is negligible, as in buildings 
which are to be protected from rain, or 
floors which are liable to splashes. In such 
cases the tar or asphalt should be renewed 
as frequently as occasion requires, as its 
waterproof action is, of course, far from 
permanent. 

COMMERCIAL WATERPROOFING 
SUBSTANCES 

It may be helpful to explain briefly how 
some of the commercial waterproofing 



substances are added to the concrete. 
Certsit is a white paste of butter-like con- 
sistency which requires to be mixed with 
an equal bulk of water and then with from 
14 to 19 further parts of water. This is 
used instead of ordinary water for mixing 
the concrete (for cement mortar, 1 part of 
the special material to 10 of water is about 
right). The concrete must be made of a 
creamy consistence, so that it can be poured 
into place. H y dr of uge- Castor is a liquid 
bituminous substance which is incorporated 
with the mortar or concrete after the wet 
mixing has been completed in the ordinary 
way. Pudlo is a fine white powder which 
requires to be mixed with the cement before 
incorporating with any of the other materials. 
The water is added to the dry-mixed 
materials through a rose, to prevent the 
powder being lifted out of the cement. 



Specifications, Quantities, Measuring, 
Estimating, and Pricing 



The Specialist System. Reinforced con- 
crete, when first introduced, was in the hands 
of a few specialists, each firm having its own 
particular system and type of bar, the latter 
being generally patented. These firms 
tendered directly for the work, and under- 
took to supply the requisite drawings and 
calculations free of charge ; on securing a 
contract, they prepared the necessary draw- 
ings, calculations, and quantities, which 
they submitted for estimates to one or 
more firms of builders licensed by them. 

This system still obtains, but it has its 
objections (a) The firm responsible for the 
design and superintendence of the work 
is commercially interested in it. (6) The 
system is inconvenient, inasmuch as it 
entails the employment of two contractors, 
whose work is interdependent, and each of 
whom may delay the work of the other. 
(c) It is not economical, as it adds an inter- 
mediate profit to the cost of the work, for, 
in spite of the specialist's claim that he 
supplies his design free of charge, it is 
obvious that he will only run his business 
as long as he makes a profit, and to do this 
he must either arrange a percentage to be 
included in the tenders of his licensed con- 
tractors, or charge sufficient for his steel to 
cover profit in addition to the large estab- 
lishment charges entailed by the employment 
of a technical staff, (d) It necessitates the 
selection of a particular system and elim- 
inates competition; but this objection has 
been partly met by the specialist submitting 
tenders from two or more of his licensed 
contractors. 

As the new construction began to find 
favour, and came into more general use, 
architects, in their desire to obtain the best 
and most economical result, adopted the 
system of inviting two or more firms of 
specialists to submit schemes and estimates, 
and so placed themselves in the position of 
having to judge these schemes on two 
bases namely, efficiency and economy by 
striking some sort of a balance, and placing 
the schemes in order of merit ; it is open to 



doubt whether many of the architects were 
in a position to take much practical account 
of the first factor, and whether the second 
factor was not usually dominant. 

A GENERAL SPECIFICATION 

It will be obvious that, assuming the 
contractors were left with a free hand (as 
they undoubtedly were in the earlier 
days), the competition was most unfair, as 
all of them were not competing on an equal 
basis. Therefore, with public departments, 
at any rate, it became the practice to issue 
with the skeleton drawings indicating the 
position and extent of the reinforced concrete 
work a specification which gradually became 
more and more definite and exact, specifying 
no detail which could apply to only one 
particular system, but setting forth all 
essential conditions and general principles 
with which all competitors must comply. 
Such a specification should fully describe the 
cement, sand, and aggregate ; the composi- 
tion of the concrete, and its method of 
mixing, whether by hand or machine, and, 
if the latter, the type of machine to be used ; 
and the description and quality of the steel 
and its ultimate tensional strength. The 
centering should be specified to be truly laid 
with close joints, to be well strutted and 
braced, and made perfectly rigid and of 
sufficient strength to support the dead 
weight of the construction as a liquid, 
without deflection. The specification should 
mention any portions to be planed and 
coated with a wash of lime and clay to 
produce a smooth surface in any position 
where the concrete is not to be plastered, 
that the beam centering is to be designed 
to allow of the sides of the forms being taken 
down first, the minimum length of time to 
be allowed to elapse before the centering 
is removed, that it is not to be removed 
without the architect's consent, and the 
length of time the beams are to be supported 
after removing the remainder of the centering. 
A full description should be given of the tests 
that will be applied for the cement, steel, etc., 



270 



SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 271 



and for the finished construction, stating 
the number and dimensions of the test 
pieces of steel and the percentage of area 
of slabs and beams to be tested, by whom 
the cost of the tests is to be borne, and the 
procedure in case they prove unsatisfactory. 

Every condition which will ensure the 
calculations of all competitors being based 
as far as possible on the same data should 
be specified ; to which end the maximum 
safe working stresses to be adopted for com- 
pression and shear in concrete, tension in 
steel (the tensional strength of the concrete 
being neglected), and for adhesion between 
concrete and steel should be clearly stated. 

The foregoing outline is given, merely to 
emphasise the principal points to be attended 
to in drafting a specification for the purpose 
of obtaining combined schemes and estimates 
in competition. It should be written in as 
much detail as possible, and embody every 
condition capable of general application, 
such as accessibility of site, means of getting 
in and hoisting materials to the various 
levels, provision of sheds for storage, etc. 
(unless these are to be provided by the 
general contractor, in which case the fact 
should be stated), protection of work from 
frost, cessation of work during frost, whether 
the contractor is to supply water or whether 
this will be supplied by the building con- 
tractor, whether he is to supply all his own 
plant and hoisting tackle, or if and to what 
extent he is at liberty to use that of the 
building contractor, and what special plant 
he must provide, etc., etc. The aim must 
be to ensure that all competitors shall 
tender on equal terms, so far as this is 
possible where each competitor is submitting 
a different scheme. 

It is also advisable to specify that the 
work must be carried on concurrently with 
that of other contractors, who are to be 
allowed all reasonable facilities for carrying 
on their work, and to state the approximate 
date at which the various portions of the 
building will be ready to receive the rein- 
forced concrete, but that the building owner 
will not hold himself responsible for any 
alteration in those dates due to delay on 
the part of the other contractors. A 
corresponding clause should be inserted in 
the building contractor's contract specifying 
these dates, and also indemnifying the 
building owner against any claim for delay 
due to the reinforced concrete contractors. 
A clause should be inserted in each contract 



stating that no claim will be entertained for 
damage to plant, materials, etc., caused by 
other contractors using the site. 

In the absence of such precautions, 
claims for delay from either contractor will 
lie against the building owner, whose sole 
remedy is, in turn, to prefer a claim against 
the offending contractor, the result of 
which is by no means as certain as is the 
necessity for meeting the costs of the action. 

The contract under the foregoing con- 
ditions being for a lump sum, it is very 
necessary to attach to the specification a 
schedule of prices for measured work to be 
adopted in variations, and prices of materials 
and labour rates to be adopted in day work ; 
these prices (except the labour rates) may 
be attached by the surveyor, and thus 
become practically a condition of contract, 
or they may be left to be priced by the 
contractors, in which case they should be 
carefully examined before accepting any 
tender. The labour rates should always be 
fixed by the surveyor, and should be the 
net current rates payable in the district. 
The percentage that will be added to these 
to cover contractor's profit, superintendence 
and "sharps" (allowance for sharpening 
tools) should be definitely stated. 

From skeleton drawings and a specifica- 
tion as outlined above, the competing firms 
prepare their detailed schemes and estimates. 
Public departments and some private archi- 
tects require calculations to be submitted 
with the estimates, a good practice when the 
architect can check them. 

QUANTITIES 

In order to frame an estmate under such 
conditions the reinforced concrete specialists 
have to prepare their own quantities, and 
this is usually done in the most rough and 
ready way. Not only are items of differing 
values lumped together, but it is open to 
question whether the actual amounts will 
bear investigation. All the concrete is 
billed per yard cube, even slabs 3-| in. or 
4 in. thick ; the centering is lumped together, 
little or no differentiation being made be- 
tween differing values ; in some cases even 
the centering for beams and slabs is lumped 
together, no height being stated for the 
strutting and bracing, and the steel reinforce- 
ment billed together in one item for each 
level, or even in one item for all levels. 
These quantities are supplied to the licensed 
firm or firms on which to estimate for the 



272 



REINFORCED CONCRETE 



actual work, and it will generally be found 
that the concrete is priced at one average 
price per yard cube, and the centering and 
steel each at an average price per yard super 
and per ton respectively ; such averages 
are rough and not exact. It has been 
argued that as reinforced concrete is a 
comparatively new material, it is useless 
to supply any more detailed quantities, as 
the builders have not yet had sufficient 
experience to price them accurately. This 
contention referred particularly to the 
centering, the one item that even at an 
early date could have been priced with 
reasonable accuracy if properly measured ; 
in any case, now that this mode of con- 
struction is becoming so general the argu- 
ment no longer holds good, and there is no 
excuse for continuing these unsatisfactory 
methods. 

As long as the present system of obtain- 
ing schemes and estimates continues, it will 
be impossible to supply complete bills of 
quantities, because, obviously, a scheme must 
be prepared before quantities can be taken 
off. Attempts have been made to prepare 
bills of quantities for the purpose of obtain- 
ing schemes and estimates with a view to 
greater accuracy and uniformity of practice 
than at present prevails, and, though their 
practical utility may be open to question, 
instructions will be given in this chapter 
for their preparation. The bills would be 
supplied to competitors, together with a 
set of skeleton drawings and a specification 
as previously described, on which to base 
the scheme ; or, better still, the specifica- 
tion might be incorporated in the preamble 
to the bill. 

Such a bill would necessarily be in skeleton 
form, and should, like the specification, be 
in as much detail and as complete as it is 
possible to make it in the circumstances. 
The preliminary bill should contain the 
requisite data for preparing the scheme ; 
it should, in fact, be a complete speci- 
fication, items which carry a money value 
being written in bill form, as for example: 
" Provide a close boarded weather-proof 
shed, with floor raised above ground, having 
a floor area of ft. super, for the storage of 
portland cement, and clear away at com- 
pletion " ; where two contractors are em- 
ployed, this item would probably be in the 
general contractor's contract. 

Procedure on the Job. Before dis- 
cussing the measured items of this bill, it 



will be advisable to consider the usual mode 
of procedure in carrying out a typical 
reinforced concrete job, as this will govern 
the descriptions, particularly in respect of 
the centering. The centering for the lowest 
floor is erected first, the reinforcing steel 
placed in position, and the concrete de- 
posited, tamped in, and levelled. When it 
has set sufficiently, sole plates are placed 
in position to distribute the weight, and the 
centering for the next floor is erected on 
them, the steel placed in position, the con- 
crete deposited, and the whole process 
repeated for the next floor, and so on, care 
being taken to place the uprights and struts 
for the centering of each floor as nearly as 
possible over those on the floor below. 
Any deviation from this order, by increasing 
the height and scantlings of the posts and 
struts, and possibly making the supply of 
materials to the lower levels more difficult, 
may tend to make the work more costly. 

Preparing the Quantities. The usual 
practice of the specialist firms in preparing 
their quantities has been to keep the whole 
of the work, centering, steel and concrete 
separate for each level, this being a con- 
venient method, which is correct in principle. 
In compiling a bill of this description, it 
must be borne in mind that, in addition to 
the actual quantity, the description must 
impart sufficient information on which to 
base the calculations ; for example, to 
determine the thickness of a slab and the 
amount of reinforcement, it is not sufficient 
to give the span only, because slabs " fixed " 
at edges, or, what amounts to the same thing, 
continuous over supports, are stronger than 
those supported only ; and slabs supported 
on one edge and fixed at the other hold an 
intermediate place between the two kinds. 
Top reinforcement must be provided over 
the supports in all cases where the slabs are 
continuous. Also, rectangular slabs, sup- 
ported or built in on four edges, may be 
reinforced in both directions; all of which 
information must be in the possession of the 
designer before he can fill up the blanks left 
for him in the bill of quantities. Where 
the positions of the beams have been deter- 
mined beforehand and indicated on the 
skeleton drawings, the dimensions of the 
beams and their reinforcement only being 
left to the designer, it is possible to embody 
the essential conditions in the description ; 
for example, in the accompanying sketch 
(Fig. 477) of a floor slab over a room or 



SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 



273 



apartment 45 ft. long by 20 ft. wide, sup- 
ported on two beams across the width of 
the room, the centre bay will be continuous 
over both supports, the end bays continuous 
over one support, and the edges in walls 
supported only ; in this case the slab might 
be billed in two separate items, each with 
its proper description ; but it would convey 
a much better idea of the requirements of 
the case to the mind of the designer if the 
whole slab were combined in one intelligible 
item, the description reading thus : "Floor 
slab at first floor level over an apartment 
45 ft. long by 20 ft. wide, the length divided 
into three equal rectangular bays, and the 
slab continuous over the two intermediate 
supports." 

Nothing less than the above description 
can, in the absence of illustration, convey 
the necessary data. 

A much simpler and at the same time 
more efficient method of conveying the 



conveniently be similar, the various slabs, 
walls, columns, beams, etc., being kept 
separate under the different reference num- 
bers or letters (their sizes being unknown 
to the surveyor). 

Slabs. The reinforcement for the slabs 
being billed by weight may be in one item at 
each level, described as " steel reinforcement 
to slabs," and a blank left for the amount. 

The centering to the slabs is billed in 
squares or yards super, the height of the 
strutting and bracing stated and the different 
heights kept separate. 

Beams. The concrete in beams is billed 
in yards cube, and may, therefore, be in one 
item at each level ; the word " cube " should 
be inserted in the proper column, the 
description written, and a blank left for the 
amount. 

The steel reinforcement to beams is 
described as such and billed by weight, a 
blank being left for the amount. 



Fig. 477. Section through Floor Slab supported by Two Beams 



information is by lettering or numbering 
the various positions on the skeleton plans 
and referring to them by corresponding 
letters or numbers in the quantities. The 
above description would then be curtailed 
to " floor slab at ' A ' at first floor level " ; 
this method enables the competitor to design 
his work from the skeleton drawings, and 
then to trace, without a moment's hesitation, 
the various items in the quantities and 
complete and price them. 

GENERAL OUTLINE OF BILL OR 
SCHEDULE 

In " taking off," the most convenient 
order to adopt will be : concrete in slabs, 
reinforcement to ditto, centering to ditto, 
concrete in beams, reinforcement in ditto, 
centering to ditto. One room or apartment 
should be completed before starting the 
next. Walls and columns should be taken 
off in similar order, being separated into 
different floor levels, and their heights 
stated. External and internal walls should 
be kept separate. 

The order in the bill at each level may 

18 



The sizes of the beams being undeter- 
mined, the girth of the beam centering is 
unknown to the surveyor, and must there- 
fore be billed per foot run and described as 
" extra materials and labour over slab 
centering to casing to beams, in. on 
soffits, and in. deep," blanks being left 
for sizes ; the different heights for strutting 
and bracing are kept separate. 

Walls. Concrete in walls should be 
billed in yards super up to 12 in. in thickness, 
the height of the wall from its floor level 
and the height of floor level stated ; blanks 
must be left in the description for the thick- 
ness. The steel and centering will follow in 
the usual order, the description given with 
the concrete conveying the necessary in- 
formation for computing the height of 
hoisting the steel and the amount of strut- 
ting to the centering. 

Columns. Concrete in columns is billed 
in yards cube, and heights stated as in the 
case of walls ; the word " cube " must be 
inserted in the proper column, and a blank 
left for the amount. The steel and centering 
will follow in the usual order. 



274 



REINFORCED CONCRETE 



Where it is decided to strengthen a wall 
by means of piers, the concrete in them 
should be billed in yards cube, the descrip- 
tion stating that it is " in wall piers " ; the 
steel and centering will follow as usual. The 
size of the piers being undetermined, the 
centering will have to be billed in feet run 
as in the case of beams, and described as 
" extra materials and labour over centering 
to flat wall surface for casing to wall piers, 
in. wide on face, and in. projection," 
blanks being left for the sizes. Where the 
arrangement and number of piers have not 
been settled it will also be necessary to leave 
a blank for the amount. 

Centering. There is some diversity of 
opinion as to the amount of detail in which 
centering should be measured, some sur- 
veyors contending that all extra labours, 
such as the intersections of beams, should be 
measured ; others measure only the super- 
ficial quantity of centering and expect the 
price to cover all incidental labours. There 
can be no doubt that the more detailed 
method is the only one that can be accur- 
ately priced, and that the less detail supplied 
the more speculative must be the pricing. 

It is scarcely possible to measure the 
centering for combined schemes and estimates 
in as much detail as though the scheme were 
already designed ; the most practical method 
of measuring for this purpose is to keep the 
items of obviously differing values separate, 
and to include all labours which cannot be 
measured in the descriptions, which should 
be as definite as possible. To avoid undue 
repetition, the method of measurement 
should be clearly stated in the preamble by 
clauses such as the following : " All items 
of centering are to include the use and waste 
of material, all labours in erecting and 
removing, and all cutting and fitting at 
angles and intersections. The measure- 
ments have been taken net as erected." 

It would serve no practical purpose to 
attempt a list of centerings for various 
positions classified according to their value, 
but taking the simplest descriptions of two 
classes, namely, to plain slabs and to 
rectangular beams, it should be a simple 
matter to separate and describe others 
which may deviate from them in any 
particular. 

Raking, cutting, and waste should be 
measured lineal. Where it is specified that 
all internal and external angles are to be 
finished with splays 2 in. wide (or as the 



case may be), these items should be treated 
as extras on the plain centering, and 
described as " extra labour forming splayed 
internal angle 2 in. wide, between wall and 
slab centering," or " between beam and slab 
centering," as the case may be, the principal 
reason for the distinction being that the 
centering at sides of beams is usually 
specified to be removable without distu bing 
the rest of the centering, in which case the 
splay would be removable with the side of 
the beam. It must not be forgotten that 
in all cases of " extras " the original measure- 
ment must include the whole surface ; for 
example, in Fig. 478 the beam centering 
would be measured the length by the girth 
from A to A, and then a lineal dimen- 
sion would be taken on both sides of the 
beam and described as " extra on centering 
to beam for forming_splayed internal angle 




Fig. 478. Gross Section through Beam 
Centering 

2 in. wide " ; to form the splay on the 
external angle an angle fillet would be fixed 
in the internal angle of the centering as 
illustrated. This should be measured and 
billed per foot run, and described as " splayed 
angle fillet 2 in. wide on the splay, and 
fixing in angles of beam centering to form 
splay on the external angle of the concrete." 
Other instances where items occur which 
might with advantage be measured as 
" extras " will be readily distinguished as 
they present themselves. 

In the case of beams with curved soffits, 
the soffits will, of course, have to be measured 
separately from the sides, but the two should 
be added together and billed in one item, 
and described as centering to beams, semi- 
circular, segmental, elliptical, or as the case 
may be, on soffits. 

Not only the centering, but the concrete 
and reinforcement should be separated into 
classes of differing values, for, in addition to 
convenience and the avoidance of repetition 
in the descriptions due to the three items 
following one another in each class, there is 
the fact that the more complex forms will 
entail more labour in placing and tumping 



SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 



27:5 



the concrete, and more elaborate reinforce- 
ment, as well as more expensive centering. 
A Detailed Bill Advisable. Seeing that 
labour is the great factor in the cost of 
reinforced concrete, and that the amount of 

,bour must vary considerably in the different 

asses of work, it is scarcely logical to argue 
that it is waste of time to differentiate 
between them, and that the contractor can 
price what is known as a " lumped " bill 
with as much accuracy as he can a classified 
or more detailed one. Although these 
more or less rule-of-thumb methods may 
have obtained in the past, there can be little 
doubt that, with the more general adoption 
of reinforced concrete construction, and the 
keener competition as firms of general 
building contractors in increasing numbers 
become skilled in its use, every opportunity 
will be taken of pricing more accurately, and 
consequently in more detail ; in short, a 
properly prepared bill of quantities for 
reinforced work will be priced with as much 
care as one for ordinary building work is at 
present. 

Such, then, are the general outlines of a 
special bill or, as it might be more correctly 
styled, schedule prepared before the re- 
inforced concrete construction is actually 
designed, for the purpose of obtaining 
schemes and estimates for different systems. 
Such a bill would undoubtedly ensure the 
estimate being made in a certain amount of 
detail and the items being more or less 
correctly measured. The bill would neces- 
sarily contain a considerable amount of repe- 
tition for instance, all the 4-in. slabs could 
not find their way into one item ; all the 
descriptions would be written by the quantity 
surveyor, but of the actual quantities, some 
would be supplied by him and some by the 
contractor, thus entailing divided responsi- 
bility, which would be most undesirable. 
Moreover, it must be obvious, from what 
has already been said, that in some cases, 
more especially those of complete buildings 
including walls, so little detail can be actually 
known of the completed scheme that it 
would be almost a farce to attempt to 
measure anything beforehand. 

An Alternative to the Bill. On the 
whole, it is open to question whether the 
preparation of a bill under these conditions 
will pay for the trouble involved, and 
whether it is not more reasonable to supply 
a specification containing a short schedule 
of instructions, naming the items to be 



measured and their method of measure- 
ment, and attaching the condition that, 
before any tender is definitely accepted, the 
contractor will be required to submit his 
tender on such schedule priced in detail. 

To give an example of a schedule here, 
or to enter more fully into the details of a 
bill, would be premature, as full instructions 
for the preparation of a bill from a complete 
set of drawings and specification are given 
later. From these, in conjunction with the 
foregoing instructions, there should be no 
difficulty in preparing a bill or schedule for 
any given set of conditions. 

"One Contractor" Jobs. It will be 
conceded that it is a great advantage to 
have the whole of a big job carried out 
by one contractor experienced in reinforced 
concrete as well as in ordinary construction. 
This avoids any division of the responsibility, 
and considerable economy both of time and 
money results. So many firms now are 
prepared to do this that there is no difficulty 
in the matter; several have "systems" of 
their own, others are licensees of one or other 
of the well-known firms of specialists, and 
all would doubtless be prepared to carry 
out any special system selected or a scheme 
specially designed by the architect or his 
engineer. 

In all cases where the particular system 
to be adopted has been decided beforehand 
the work can, of course, be properly measured 
and incorporated in a bill of quantities, and 
if it is decided to invite only builders pre- 
pared to do the whole of the work, then one 
bill of quantities will suffice ; but the whole of 
the reinforced concrete work, including the 
steel and centering, should be billed as a 
separate trade, and not split up and em- 
bodied in the ordinary trades of " concretor," 
" carpenter and joiner," and " smith and 
founder." 

DESIGNS INDEPENDENT OF THE 
SPECIALIST 

The ideal method of dealing with re- 
inforced concrete work is for the architect 
to design his own or employ an independent 
engineer to do so, being then free to use any 
section of bar considered best for his par- 
ticular purpose, or any of the patent bars 
(provided the patentees would supply them), 
or even to use two or more different bars in 
the same building, choosing them for their 
fitness for particular positions and uses. 
Some, at any rate, of the patentees are 



276 



REINFORCED CONCRETE 



already prepared to supply their steel under 
these conditions, and if the system became 
in any degree general all would be obliged 
to fall into line. 

The great obstacle to the adoption of such 
a system is the fact that comparatively few 
architects can design reinforced concrete 
work nor can they reasonably be expected 
to do so, as it is purely engineering work, 
foreign to the architect's nature ; why, how- 
ever, an architect should not employ an 
independent engineer, instead of placing 
himself in the hands of commercial firms, 
is not quite so easy to see, except for the 
difficulty of explaining the situation to the 
client, and inducing him to pay the fee, and 
the reluctance of the architect to paying it 
out of his own pocket. It cannot be too 
strongly impressed on clients, however, 
that the fee must be paid in any case, 
either as such or as extra price of materials. 
For example, take a case where reinforced 
concrete is used for all floors, beams and 
flats in which the total cost of the reinforced 
work, exclusive of all ordinary builders' 
work, is 1,000 ; assume that 450 cwt. of 
steel (a reasonable average case) will be 
employed. This steel would be supplied to 
the builder at, say, 14 per ton (this is one 
of the actual rates), to which he would have 
to add the cost of hoisting and placing in 
position and profit, an average of 25s. to 
30s. per ton. Now, 8 per ton would be a 
very good price for ordinary steel rods, or, 
allowing for extra labour in bending and fixing 
stirrups, say 9. The addition of 1 10s. 
to this ought to be sufficient to cover the 
extra value of any patent bar. Assuming, 
then, for the sake of argument, 10 10s. as 
a proper market price for the patent bar, 
instead of 14, a surplus of 3 10s. per ton, 
or a total of 78 15s., equivalent to 7| per 
cent, on the total cost of the reinforced 
concrete work including the centering, will 
remain to pay for drawings, superintendence, 
etc., by an independent engineer. 

Reinforced concrete being as much a 
mode of building construction as is a system 
of brick walls with wood or steel joists, it 
is difficult to believe that architects will 
continue to ignore its study ; in any case, 
there can be no reason why public depart- 
ments should not see that a section of their 
large technical staffs is skilled in this branch 
of work, and so make themselves independent 
of commercial firms as far as design and 
calculations are concerned. 



MEASURING 

For the purpose of measuring and estimat- 
ing, at any rate, it will be necessary to assume 
a settled and complete design before the 
system of measurement can be explained. 

There are many instances, particularly, 
perhaps, in England, where reinforced con- 
crete construction is favoured for floors, 
flats, roofs, partitions, columns, and beams, 
where its use for external walls would not be 
considered. In such cases there will be 
several items of builders' work in connection 
with the reinforced concrete; where there 
are two contractors these items will be 
executed by the building contractor, where 
only one contractor is employed these items 
must still be measured, but should be billed 
in their proper place in the ordinary trades, 
and not under the heading of " reinforced 
concrete " ; these items include such as 
"cutting or leaving chase -- in. deep for 
edges of floor slabs and making good," 
" cutting or leaving recesses for ends of 
reinforced concrete beams and padstones 
(here state dimensions), and building in 
in cement," etc., etc. ; similar items occur in 
the case of ordinary concrete and steel joist 
construction, and therefore they will not be 
further referred to here, it being assumed 
that the reader is already acquainted with 
the modes of measurement of ordinary 
builder's work. 

As already stated, it is the practice of 
specialists, when taking their own quantities, 
to measure all concrete cube ; while this 
practice is here deprecated, no undue or 
unnecessary elaboration will be advocated, 
but the builder should be given the oppor- 
tunity of judging what class of work he is 
estimating for. The fact of a contractor 
pricing two or more separate items at the 
same rate is not necessarily a criticism on 
the work of the surveyor ; the contractor 
should be given the opportunity of judging 
their relative values for himself, and it is 
not a good practice for the surveyor to do 
it for him. Again, the contractor may not 
have considered these items of equal value, 
but may have priced them at an average 
value after noting the ratio between the 
quantities of the different descriptions, in 
which case, assuming a variation on the 
contract entailing an addition to the more 
expensive and an omission on the cheaper 
description, the contractor would, under the 
usual terms of a building agreement, stand 
to lose, as all variations would be valued at 



SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 277 



the rate stated in the bills ; he has, however, 
been provided with the requisite data to 
enable him either to price separately, or to 
form a true average, whereas, when items 
of differing values are combined under one 
description, the contractor has no option 
but to affix an average rate, and a specula- 
tive one at that. 

There is every reason to suppose that 
reasonably detailed quantities and increasing 
competition will gradually lead to more 
careful and accurate pricing. 

Measuring Concrete. The concrete in 
walls and partitions up to 12 in. thick 
should be measured super, also rafts up to 
12 in. in thickness. Over this thickness, 
these items should be measured cube ; con- 
crete in foundations, columns, piers and 
beams should also be measured cube. The 
work should be kept separate at the different 
levels ; floor levels will form convenient 
divisions, but their height above ground 
should be stated. The different classes of 
work should be kept separate ; for example, 
slabs on the slope (as to pitch roofs, etc.) 
should be kept separate from horizontal 
slabs. Note that slabs laid to a pitch of 45 
or less may be laid on single centering, but 
that to steeper pitches the concrete will have 
to be packed between centering; the two 
classes of work must be kept separate as the 
labour is different. The descriptions of the 
two classes respectively would be somewhat 
as follows : " 5 in. concrete slab laid sloping," 
and " 5 in. concrete slab sloping and packed 
between centering." Arched beams should 
be kept separate from straight ditto ; 
circular walls and partitions separate from 
straight ditto, and the radius stated. 
Vaults should be measured super, stating 
whether segmental, semicircular, etc. ; and 
thickness given. Domes should be measured 
super, stating the thickness, unless this 
varies, in which case it will be necessary to 
measure them cube, stating that it is in 
domes, the thickness varying from to 
(the superficial area of a dome is the same 
as that of the circular portion of a cylinder 
exactly enclosing it). Concrete in curved 
ribs should be measured cube, and described 
as in semicircular or segmental ribs, or as 
the case may be, and the radius stated. The 
extra concrete forming internal splays to 
angles may be measured either per foot run 
and sizes given, or per foot cube and de- 
scribed ; the latter method is preferable, as 
the concrete is only worth the same rate as 



that in the wall or beam to which it is 
attached, the extra labour being on the 
centering. 

In measuring concrete, no deduction is 
made for the space occupied by the re- 
inforcement. 

Small Holes. Small holes required to 
be left in the concrete for the passage of 
pipes, etc., should be numbered, and their 
size given, the wood plugs or drums fixed 
on the centering being included in the 
description of the holes. 

Every precaution should be taken against 
having to cut into the concrete after it is 
laid. It is better and cheaper to bed bolts 
with back plates in the concrete, and leave 
them projecting the requisite distance for 
fixing. These bolts have to be fixed in 
position in the centering. The bolts them- 
selves are usually provided under the 
building contract, but they must be numbered 
for the labour of "bedding in." To cut a 
hole for a bolt or any other purpose in 
reinforced concrete is an expensive item 
of labour, the material being so very hard. 

Measuring Steel Reinforcement. The 
steel reinforcement is measured per foot 
run and billed in cwt., qr. and lb., billing to 
the nearest 1 lb. ; for example, 54 lb. would 
be billed as 2 qr. ; 60 lb. as 2 qr. 1 lb. ; 59 lb. 
as 2 qr. ; 73 lb. as 2 qr. 14 lb., and so on. 

Care must be taken in measuring the steel 
to add for all laps and, unless these are 
clearly shown on the details, the necessary 
information must be obtained from the 
designer. The weights of the bars per foot 
run may, in the case of standard bars with 
rigidly attached web members, be obtained 
from the trade catalogues ; in the case of 
loose stirrups, these will have to be separ- 
ately measured and weighted out ; they 
should be measured from an actual sample 
bar, in order to obtain the correct length 
around the bends or twists ; they may then 
be weighted out from tables of weights or 
some sample 12 in. pieces actually weighted ; 
in the case of standard stirrups of any 
particular system the weights may either 
be obtained from the trade catalogue or by 
actual weighing ; stirrups may either be 
measured per foot run or numbered, which- 
ever lends itself the more readily to the 
subsequent weighting out ; in the case of 
bars and rods it is only necessary to remember 
the weight per foot super of 1 in. plate, and 
the weight per foot run of 1 in. diameter 
rods to obtain the required weight of any 



278 



REINFORCED CONCRETE 



other sizes ; for example, the weight of 
1 in. by 1 in. bar per foot run will be r \th 
that of 1 in. plate per foot super ; that of 
1 in. by 2 in. th ; that of 1 in. by ^ in. 
Jjth and so on ; in the case of rods, seeing 
that the areas of circles vary as the squares 
of their diameters, it will only be necessary 
to multiply the weight of 1 in. rod by the 
square of the diameter of any other size to 
find its weight ; thus the weight of a in. 
diameter rod will be th that of 1 in. ; 
that of a 2 in. rod four times that of 1 in. ; 
and so on. 

The steel reinforcement should be kept 
separate for the different positions and 
classes of work. In a job where bars with 
fixed web members, such as the Kahn, are 
used and it is necessary in certain portions of 
the work to use loose stirrups, this work 
should be billed separately and properly 
described, there being considerably more 
labour in placing steel bars and loose stirrups 
than in placing a bar having fixed stirrups. 

Measuring Centering. Centering is not 
only a very important item, but also a 
very costly one. Moreover, the value of the 
different classes of work varies so consider- 
ably and extra labour forms such an appre- 
ciable item that it should be measured as 
carefully and in as much detail as the cir- 
cumstances of the case permit. There have 
probably been more surprises at the end of 
a job due to the speculative nature of the 
items of centering than to any other cause. 

" Use and Waste." The concrete and 
the steel can be priced with reasonable 
accuracy, even when they are not taken in 
as much detail as they should be, but not 
so the centering. That must be, unless 
carefully measured in detail, a very specu- 
lative item. The materials are charged as 
use and waste only, but the centering has 
to be very carefully erected, and labour 
enters largely into the cost. No definite 
proportion can be fixed for " use and 
waste," as this differs so widely in different 
classes of work ; in the case of centering to 
a plain floor slab, there will be a certain 
amount of waste in cutting to the required 
lengths, but nearly the whole can be re-used 
by further cutting on another job, or possibly 
in another position on the same job. The 
larger the areas of the slabs the less waste 
will there be. The labour, also, in erecting 
slab centering is comparatively simple. 
But in beam centering there will be much 
more cutting and \vaste and more labour. 



As the dimensions and span of beams 
decrease the waste and labour increase ; 
in elaborate works, much of the centering 
will scarcely be worth the cost of carting 
from the site, and though still technically 
described as " use and waste," its full price 
ought to be included in the estimate. 

First-class Centering. Headers will 
.know that a perfectly true and finished 
surface on walling and mouldings, and even 
on carving (so called), provided there is no 
undercutting, can be left direct from the 
centering or moulds without any subsequent 
rendering ; but for this purpose the false- 
work must be wrought on one side and 
jointed up as a first-class piece of joinery, 
the carved work being cut in reverse in 
wood blocks, which are inserted in their 
proper position in the wall centering. Such 
centering will, of course, be far more ex- 
pensive than the ordinary style, and more 
labour and care will be entailed in packing 
the concrete. 

Centering required to be planed and 
covered with a wash of lime and clay to 
leave a smooth surface on the concrete may 
be kept separate and described, but it is 
preferable to measure the planing and coat- 
ing separately. 

In order to form a key for plastered ceil- 
ings, it is sometimes specified that a layer 
of fine breeze is to be spread on the slab 
centering before filling in the concrete, this 
should be measured separately and billed 
in yards super. 

Centering for a scheme already designed 
can and therefore should be measured in 
more detail than when measuring for com- 
bined schemes and estimates. 

" Centering " Clauses in Preamble. In 
the case of ordinary building works, it will 
often be impossible to separately measure 
the bearers, struts, and braces to the center- 
ing, as it is unusual to supply drawings for 
this, but any clauses in the specification 
governing the design and erection of the 
centering should find their way into the 
preamble. In the absence of any more 
definite specification, some such clause as 
the following should be inserted in the 
preamble : " The centering to be well 
strutted and braced and made perfectly 
rigid and strong enough to sustain the weight 
of the slabs, etc., as a liquid mass, without 
deflection." This clause, used alone, throws 
the responsibility for the efficiency of the 
centering on the contractor. To avoid 



SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 



279 



repetition in the descriptions, the following 
clause may be inserted in the preamble : 
" The centering to include all bearers, struts, 
braces, etc., wedging, and all ironwork 
required unless otherwise stated " ; it will 
then only remain to state the purpose of 
the centering and the height of the strutting 
in the description. 

Specially Designed Centering. Where, 
as in the case of heavy engineering works, or 
from the special nature of the work, the 
engineer or architect elects to design his 
own centering it will be advisable to mea- 
sure it in detail as designed, otherwise it 
would be difficult to enforce the work being 
carried out to drawings without incurring 
an extra. When measuring strutting and 
bracing in detail, it is advisable to measure 
the timbers lineal and to state their scant- 
lings, rather than to measure them cube 
as in the case of ordinary carpentry. The 
architect's permission should be obtained 
to insert a clause in the preamble to the 
effect that the scantlings given for the 
timbers to the centering are minimum 
ecantlings, but that the contractor is at 
liberty to use other scantlings of equal or 
greater strength with the architect's ap- 
proval, provided no extra cost is incurred. 
The object of this clause is to enable the con- 
tractor to use such scantlings as he may have 
in stock or as are most easily procurable, so 
as not to increase unnecessarily the cost of 
the temporary work. 

Centering should be measured super 
(except as hereafter stated) and described, 
and the height of the strutting given ; that 
at different levels and to different descrip- 
tions of work should be kept separate. 

Openings. All openings should be de-' 
ducted where it is practicable and convenient 
to omit the centering, but in the case of 
small openings, particularly in horizontal 
slabs, it will often be cheaper to run the 
centering through than to provide extra 
supports or trim the bearers round the 
opening, in which case it should not be 
deducted, but the vertical form to edges 
should be erected on it. It will be necessary 
to keep some openings clear for access and 
working purposes ; staircase and lift open- 
ings are usually the most convenient for this 
purpose in the horizontal slabs, and door 
openings in the walls. In all cases where 
the centering is deducted, an item should 
be taken of "cutting and waste around 
openings " ; with regard to doors and windows 



these may be numbered and the average 
size of openings given, stating whether one 
or both sides are measured, or the dimension 
may be taken run, and a similar dimension 
taken on slabs around openings for stairs, 
skylights, etc., etc. Keep the centering to 
edges of openings in narrow widths separate. 

The vertical centering around openings 
in slabs should be described as such, and 
measured super ; where 9 in. or less in 
height, it may be measured run and the 
height given. 

Centering to Walls. Vertical centering 
to walls and partitions should be measured 
to both sides ; it is not a good practice to 
measure one side only, stating in the descrip- 
tion that " one side is measured for two," 
because the amount often varies considerably 
on the two sides of a wall, especially if it be 
an external wall with cross walls abutting 
against one side only. 

Centering to external walls should be 
kept separate. There are not quite the 
same facilities for getting at both sides of 
the centering and the upper stories can only 
be strutted from one side. 

Moulds for Architectural Features. 
Where centering has to be formed into 
moulds for architectural features it should 
be kept separate, as more labour and waste 
is involved. It may either be separately 
measured and the vertical wall centering 
behind it deducted or measured as extra 
on the vertical centering ; in most instances, 
the latter method is to be preferred. A 
typical case is that of a shaped core for a 
moulded cornice to be finished in cement, 
the simplest and probably the best method 
of measuring which is per foot run as extra 
material and labour over vertical wall 
centering, stating in the description both 
the height and the girth of the centering, 
so as to enable the estimator to ascertain 
exactly the extra net amount of material 
involved and to judge the extra labour and 
waste. By making it a lineal dimension, 
there is also the added advantage that the 
mitres, etc., may be "written short" in 
the bill. It should be particularly noted 
that this item is described as extra materials 
and labour; this makes it perfectly clear 
that no deduction has been made from the 
vertical centering for that portion imme- 
diately behind the cornice ; nor, on the 
other hand, has any material been previously 
measured to cover the projection. 

Centering to Inclined Slabs. Centering 



280 



REINFORCED CONCRETE 



to inclined slabs (as for pitched roofs, etc.) 
should be kept separate from horizontal 
slabs. Centering to slabs inclined at a 
steep pitch (more than 45) requiring 
centering both sides should be kept separate ; 
in this case the description must be carefully 
worded to avoid any ambiguity, thus : 
" Centering to both sides of inclined slabs 
(both sides measured)." 

Centering to Beams. Centering to 
beams should be measured super and the 
different descriptions of beams kept separate ; 
as previously explained, the super is obtained 
by taking the length by the girth of the 
centering on the outside of the sheeting. 
Internal and external chamfers on beams, 
etc., should be measured as has been pre- 
viously explained on p. 274. Intersections 
of beam centering with beam and of beam 
centering with wall or padstone centering 
should be measured where the end of one 
beam abuts on another beam, padstone or 
concrete wall. A piece has to be accurately 
cut out of the centering of one of the beams 
or the wall, and the end of the abutting cen- 
tering accurately and closely fitted thereto. 
The most comprehensive way of measuring 
these items is to number and describe them, 
stating the girth of the centering of the 
abutting beam ; see example_ below : 



cleanly from the concrete should be either 
included in the description or separately 
measured. 

Moulds for Cornices, etc. The moulds 
for cornices, string courses, etc., should be 
measured lineal as extra on the vertical 
centering, internal and external mitres, and 
mitred and returned ends numbered and kept 
separate. Any ends fitted against plain 
centering should also be numbered, such as 
an internal angle where the cornice is not 
returned, in which case the vertical centering 
would have to be cut and scribed to the 
outside of the cornice moulds ; these 
numbered items should be " written short " 
in the bill. Blocks for carving should be 
numbered and described, and the size stated. 
The actual carving of the blocks must be 
treated as a labour item, and is best provided 
for by a provisional sum, unless it is of a 
geometrical or other simple nature such as 
could be dealt with by a description or 
sketch. The typical example on the next 
page will serve to illustrate the descriptions 
and method of billing. The reference letter 
" A," in addition to the purpose it serves 
in this section of the bill, will be found even 
more useful in saving repetition in the 
descriptions of similar centering at other 
levels. 



No. 



Intersections of beam centering 27 in. \ 

girth J 

-Ditto ditto with wall centering \ 

30 in. girth J 

Ditto ditto with padstone cen- \ 
tering 24 in. girth / 



8. 



Centering to Padstones. This should 
be measured super and kept separate. 
Centering to Columns and Piers. 

This should be measured super, each descrip- 
tion, such as " to columns," " to piers," and 
" to wall piers," etc., being kept separate and 
properly detailed. They should be billed 
in their proper order of floor levels, and their 
heights stated. All junctions of columns or 
piers with slab centering should be numbered 
and their girth stated. 

Centering to Produce Fine Finish. 
Centering, such as that described on p. 278 
to produce a finished face on the concrete 
direct from the moulds, should of course be 
kept separate and carefully described ; any 
coating of the centering or other device 
to allow of the centering coming away 



ABSTRACTING 

In many cases an abstract will scarcely 
be necessary, but in others it may be found 
more convenient to have one. It is assumed 
that the reader is acquainted with the 
methods of measuring, abstracting and 
billing ordinary builder's work, and it will 
therefore be unnecessary to do more here 
than to suggest a convenient form for 
abstracting reinforced concrete work. A 
perfect abstract should present the items 
in the correct order for billing ; it has been 
stated before that it is the custom of special- 
ists to group the three main items of concrete, 
steel, and centering in sections according to 
levels. Floor levels are adopted as far as 
possible, as being convenient divisions. 
This method, although preventing all the 






SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 281 



Yds. 
200 



Ft. 



100 



In. 



Sup. 



Run 




Centering to outer face of external 9-in." 
walls from first to second floor levels, the 
boards tongued and grooved and carefully 
jointed and clamped, the inner face to be 
planed to a true and even surface, so as to 
leave no marks of this jointing in the surface 
of the concrete 

Extra materials and labour over vertical'' 
centering as " A " for centering to moulded 
cornice 24 in. girth 12 in. high as sketch, 
carefully jointed to the vertical centering ; 
all joints to be close and to be grooved 
and tongued, and the internal surface of 
the mould clean and true, so as to leave 
no markings on the concrete 

No. 5 Internal mitre 

No. 10 External mitre 

No. 2 Mitred and returned ends 

No. 1 End fitted to vertical centering andl 
the vertical centering cut and scribed / 

No. 3 3 in. blocks, size 2 ft. in. by 3 ft. 6 in., 
carefully jointed up and prepared for 
carver, and cutting out for and inserting 
and carefully jointing same to vertical 
centering as " A." (Vertical centering not 
deducted.) 



s. d 



items of either material being billed con- 
secutively, places each in its proper rela- 
tion to the others in a very comprehensive 
manner, and is to be commended for the 
main items. It is advisable for the sake of 
clearness to apply the system to the main 
items only, and to bill all incidental items 
and labours under a further sub-heading of 
" Sundries," which will comprise more 
particularly such items as are not affected 
by the level at which they are executed. 

The setting out of a portion of an imag- 
inary abstract on pp. 282 and 283 will illus- 
trate what is meant. 

BILLING 

When one contractor is to carry out the 
whole of the work, including the reinforced 
concrete, all preliminary works, whether in 
connection with the general or the reinforced 
work, would find their way into the one 
preliminary bill. Many of the items, such, 
for example, as " water," would be covered 
by the usual clause, and would obviously 
apply to the whole of the work. 

When the reinforced concrete work is to 
be made a separate contract, it must have 
its own separate bill of preliminary works, 
and it is often necessary to refer from one 



contract to the other in order to make it 
clear to each contractor exactly how much 
is expected of him and how much the other 
will have to do, as, for instance, in the 
provision of water, scaffolding, hoisting 
tackle, etc., previously referred to (see 
p. 271). 

The Preamble. Having written the 
" Preliminary Bill," the " Bill for Reinforced 
Concrete Construction " must in either case 
start with a " Preamble," most of which 
will be taken direct from the " specification." 
Judiciously worded clauses in the preamble 
will often save repetition of descriptions in 
the bill. The system of reinforcement to 
be adopted should be stated in the preamble, 
as the amount of labour in placing the steel 
varies with the different systems. Where 
it is found necessary in a few positions to 
adopt a special system of reinforcement 
the words " except where otherwise stated " 
should be added after the description of 
the system, and the special description of 
the exceptions given opposite the items. The 
nature of the clauses forming the preamble 
is indicated when treating of specifications, 
but clauses which relate solely to design 
will, of course, be quite superfluous. The 
following example will serve as a guide, but 



282 



REINFORCED CONCRETE 



*/- q. &<* 



<Mit 6<*~, //<>'. 



.,- 

JUU 





* ~?"?\ ?*' ' 
'7' 



it must be added to, and altered as 
necessary to meet the special re- 
quirements of each case : 

No. 2 REINFORCED CONCRETE 

The reinforced concrete construction 

will be on the system (except where 

otherwise stated) and the whole of the 
steel is to be obtained from 

The materials including the cement are 
to be tested as hereafter specified and 
the cost of all such tests whether ac- 
cepted or not are to be borne by the 
Builder. 

The cement to be portland from an 
approved manufacturer to be slow setting 
in accordance with the British Standard 
specification and capable of meeting the 
tests therein specified. 

The sand to be river or pit sand 
coarse and gritty with grains of various 
sizes and perfectly clean. 

The aggregate to be composed of clean 
gravel carefully selected and screened to 
pass a J-in. mesh. 

The concrete to be 'composed as fol- 
lows : 

Gravel 27 cub. ft. 

Sand 13J cub. ft. 

Cement 5i cwt. 



to be thoroughly mixed in a mechanical mixer 
approved by the Architect. 

Steel must have an ultimate tensile strength 
of not less than 28 tons or more than 32 tons 
per square inch with a contraction of area at 
fracture of not less than 40 per cent, the fracture 
not to show more than 10 per cent, of granular. 
Samples of the steel bars must be able to stand 
being bent cold until the ends close over a bar 
of one and a half times the diameter of that of 
the bar to be tested. 

The Architect or his representative will select 
and stamp such samples as he may consider 
desirable at any time either at the steel works 
or on the site and the Builder will be required 
to send such samples carriage paid to Messrs. 

or other approved testing and 

experimenting works and the reports obtained 
shall be accepted by the Builder. 

No welding of the rods or bars will be 
permitted. They are to be bent 
cold. 

The steel is to be brought upon 
57"^ the site as required and no steel is 

to be fixed in a dirty condition. 

The steel reinforcement to slabs 
is to be protected by a minimum thickness of 
J in. and that to beams by a minimum thick- 
of U in. c 



c. 



. . 



. 

/o - a ' 




SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 283 

total area) as the Architect 
shall direct in the following 
manner : A dead load equal 
to the calculated super load 
plus 50 per cent, shall be 
evenly distributed over such 
parts of the floor etc. as are 
to be tested and the deflec- 
tion shall in no case exceed 
sinjth part of the span of the 
slab or beam under test. The 
extra load of 50 per cent, 
shall be removed immediately 
Q after the reading and the cal- 

Vy*^/^. r~-*_ cu i ate( i \ oa ^ sna n b e left for 

at least 12 hours in position 

The workmanship throughout is to be of the when it shall be removed ; fresh readings taken 
very best description and to the entire satisfac- 24 hours later shall record no appreciable set. 
tion of the Architect who shall have full power No cracking or scaling of the concrete shall be 
to have any parts of the work which are in his developed under any test. ft 

opinion unsatisfactory re-executed to his satis- In the event of the aforesaid deflection being 
faction and the Builder is to bear the expense. exceeded or an appreciable permanent set being 

The centering to be true and rigid and of found to remain or of the results of the test of 
sufficient strength easily to carry the dead weight any portion being otherwise unsatisfactory to the 
of the construction as a liquid without deflection ; Architect that portion at the expiry ofjsix 
the posts or struts to be 
properly braced in both 
directions. All joints to 
be tight so as to prevent 
leakage. The centering 
to be so designed that 
the sides of the beams 
can be taken down first 
then the slab centering 
complete. 

No centering is to be 
removed in less than four 
weeks' tune after con- 
creting or until the con- 
crete has thoroughly set 
and has aged to give it 
sufficient strength to 
carry its own weight and 
in addition whatever live 
load is liable to come on 
the work during the 
course of construction. 
Beams are to remain 
supported at least two 
weeks after all other false- 
work has been removed. 
Columns shall not be 
given their full loading in 
less than five weeks 
after concreting. In no 
case shall any centering 
be removed without the 
consent of the Architect. 

The Builder shall at 
his own expense test such 
parts of the work (not . 

being more in the aggre- *~ ' a *y * J 
gate than 5 per cent, of the 




284 



REINFORCED CONCRETE 



weeks from the date of the first test shall be 
again tested in a similar manner. Should 
further deflection take place during the second 
test or further permanent set be found to exist 
or should the work in the opinion of the Architect 
still prove unsatisfactory then and in such case 
the Architect shall have full power to require 
the removal of the unsatisfactory portion and 
its reconstruction in a satisfactory manner at 
the expense of the Builder. 

The test load shall not be applied to any part 
of the works until a period of two calendar months 
shall have elapsed since the deposit of the 
concrete in situ. 

No cranes or other machinery shall be placed 
upon the reinforced concrete work and the 
extent to which the beams floors etc. may be 
loaded with materials during the progress of the 
work shall be subject to the approval of the 
Architect. 

The price of the steel is to include for all 
labour in bending and fixing. 

Other Clauses in the Preamble. 

The preamble to a bill should include all 
clauses from the specification which may in 
any degree affect the prices of the measured 
items in the bill, as well as any general 
descriptions of material or labour which may 
save repetition in the descriptions of the 
measured items. It is not usual to include 
in the preamble any items requiring a price 
to be attached and for this reason in a bill 
of quantities for ordinary builder's work 
it has been the custom to place any clauses 
for testing work in situ (as, for example, in 
the case of the plumber's bill) at the end of 
the trade and to start the item with the 
words " Allow for testing, etc., etc." 

The Testing Clauses. The testing 
clauses in a reinforced concrete bill are not 
at present usually priced, but in a large job 
they must carry an appreciable money 
value which cannot reasonably be spread 
over the various items in the bill ; the 
proper method of dealing with this item 
would be to put the clauses describing the 
method of testing in the preamble as in the 
foregoing example, and to insert as the last 
measured item in the bill the following : 
" Allow for testing the reinforced concrete 
construction as described." The contractor 
will then be at liberty to price this item or 
not, as he thinks fit. As competition gets 
keener, and with more detailed bills to help 
him in pricing each item as closely as possible, 
it may well be that he will embrace the 
opportunity of attaching a separate price 
to the testing. 



Examples of Billing Measured Items. 

It is always advisable in billing to adopt 
some definite order, but it is by no means 
essential to adopt that suggested in the 
section on " Abstracting." The descrip- 
tions, however, must be absolutely clear 
and definite so as to leave no doubt in the 
mind of the estimator as to what is meant, 
and they should also be as concise as is 
consistent with this rule. The example 
below illustrates the suggested system of 
billing the measured items following imme- 
diately after the preamble. 

Then proceed to bill the next floor in a 
similar manner, and so on, floor after floor, 
finishing with the roof or flat. Having 
billed the whole of the principal items in 
this manner, proceed to bill the incidental 
and labour items under separate sub-head- 
ings of " Sundries in Reinforced Concrete," 
and " Sundries in Centering" (see p. 215). 

These two examples illustrate the prin- 
ciples to be followed in billing, and from 




/to 



^ cjL^u^-^f * * <^*6^v 7 

/IT- & I 

Hi^.f <* 'feTOWK-ty J 



S*8-e 




SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 



285 



far 




*.. 



be assessed after considerable experience 
and close observation ; and there are no 
reliable data at present available for general 
use. 

In the case of a standard bar with fixed 
shear members, the relative value of fixing 
in different positions will vary comparatively 
little, but the value of hoisting will, of course, 
vary with the height. 

The centering, if properly measured, can 
be priced with reasonable accuracy, and a 
little experience should enable the estimator 
to judge the relative proportion to be adopted 
for use and waste in different positions. 

Prices for Concrete. Concrete com- 
posed of 5 cwt. of cement, 13| cub. ft. of 
sand, and 27 cub. ft. of gravel, is equivalent 
to a proportion of 1 : 2 and 4 by measure ; 
a cubic yard of this finished concrete requires 
nearly 1 cubic yards of dry materials, 
say 40 cub. ft., which, divided up into the 
given proportions (1 : 2 and 4), would give 

cub. ft. of cement, = 

7 7 

sand and 



them the student should be able to bill and 
describe any other items, always bearing in 
mind that the object of a bill of quantities 
is to convey to the mind of the estimator in 
the most concise manner possible exactly 
what the builder has to do and how much 
of it. 

A schedule of rates to be paid in day- 
work similar to that described on p. 271 
should be given at the end of the bill. 

PRICING 

With regard to pricing the estimate, it is 
only necessary to examine a dozen tenders 
for the same work to see that pricing is not 
an exact science, and therefore any prices 
which may be given can only be approximate 
or average prices. The method of arriving 
at the total value of each item is by analysis, 
making due allowance for any special 
circumstances peculiar to each job. The 
total quantity of reinforced concrete will 
determine whether it will be more economical 
to mix the concrete by hand or to employ a 
mechanical mixer (if this has not been 
already settled by the architect). 

The value of a cubic yard of concrete of 
any given composition will remain more or 
less constant, and may be readily ascertained, 
but the relative value of hoisting, depositing 
and tamping in various positions can only 



cub. ft. of 



cub. ft. of gravel, = 5f 



cub. ft. of cement at 90 Ib. per cub. ft. 
weighs 514 Ib. ; these data, together with 
the prime cost of the materials, including 
cartage to the site and the cost of mixing, 
will give the prime cost value of a cubic yard 
of concrete. The cost of water will be 
covered by the item for water in the pre- 
liminary bill if this item has been priced ; 
if, however, the contractor has only attached 
a price to this item to cover fees and the 
cost of the temporary plumbing, the concrete 




286 



REINFORCED CONCRETE 



will have to bear its proportion of the water 
charges. In cases where the water is paid 
for by meter, about 30 gallons should be 
allowed per cubic yard of concrete. The 
cost of mixing will depend on whether it is 
mixed by hand or by machine. The cost of 
labour in hoisting, placing, and tamping 
will depend upon the height of the hoisting, 
and whether the work is in walls, slabs, slop- 
ing roofs, beams, etc., etc. To the cost of 
the concrete in situ so obtained must be 
added a percentage to cover builder's profit 
and establishment charges. The following 
example, to which average London prices 
have been attached, will explain the principle 
of analysis of a cubic yard of concrete : 

s. d. 
Portland cement, 514 Ib. at 30s. per 

cement ton delivered . 7 

Sand, 11 f cub. ft. at 6s. 6d. per 

cub. yd. ... 29 

Gravel, 22 cub. ft. at 7s. per cub. yd. 5 11 
Labourer measuring and mixing, 

3 hrs. at 7d. . . . J. 9 

Prime cost of a cub. yd. of concrete 

exclusive of hoisting and placing 17 5 

To this price must be added the value of 
hoisting, placing, and tamping in various 
positions, and a percentage to cover profit 
and establishment charges. For example, 
a yard super of floor slab 6 in. thick will take 
Jth of a cubic yard of concrete, and the 
analysis will be as follows : 

s. d. 
Jth of a cubic yard of concrete as 

above 2 11 

Concretor placing and tamping, and 
Labourer hoisting not exceeding 
20 ft., and attending upon con- 
creter 4 

~3 3 

Profit and establishment charges 

15 per cent. 6 



In a similar manner may be found the 
price of a floor slab of any other thickness 
or at any height, or the price of any item of 
concrete, whether billed in yards super or 
yards cube ; the prime cost of a yard cube, 
as given above, is the basis in every case. 

The above analysis is for hand-mixed 
concrete. If a machine mixer is used, the 
cost of the motive power per hour, and the 
wages of the attendants must be ascertained, 
to which must be added a proportion of the 



cost of installing the plant, and a percentage 
on the capital cost of same to cover wear and 
tear ; from the cost of running the mixer 
thus found, and the amount of concrete 
which it is capable of turning out per hour, 
the cost of mixing per yard cube is readily 
ascertained. 

Prices for Steel Reinforcement. 
The cost of steel for the reinforcement, 
delivered on the site, will depend upon the 
bar used, and the distance for carriage and 
cartage ; to the cost of the steel as obtained 
from the makers, and carriage, must be 
added the cost of labour in hoisting and 
placing according to height and position ; 
the amount of such labour will vary with 
the system adopted, from the minimum 
amount in the case of bars with fixed shear 
members to the maximum in the case of 
ordinary rods where the shear members have 
to be bent to pattern and fixed in position. 
The Kahn trussed bar is an example of a 
bar with shear members in one piece with 
the bar. 

The bar is rolled with side flanges which 
are machine cut ready to be turned up at 
an angle of 45 to form the shear members ; 
the price of the patent bar is about 14 
per ton, or 14s. per cwt., at London station, 
to which must be added the cost of cartage 
to site, hoisting, fixing in position, and profit 
and establishment charges. The total cost 
of the bar, in London, fixed in position, 
averages for floor slabs and rectangular 
beams supporting same 15s. 3d. per cwt. ; 
for padstones at ends of beams, 15s. 6d. ; 
for ground floor walls 15s. 

Prices for Centering. The cost of 
materials for centering remaining constant, 
the value of centering in any given position 
will be governed by the proportion to be 
allowed for use and waste, the amount of 
labour involved in that particular position, 
and the quantity of bearers, struts, and 
braces per unit of area. Centering to floor 
slabs 12 ft. high will require, on an average, 
about 18 cub. ft. of timber for bearers, 
posts and braces, and the cost of materials 
for a square of centering will be as follows : 

s. d. 
1 square of 1 in. boarding at 13s. per 

square 13 

18 cub. ft. of timber in bearers, posts, 

etc., at Is. 7d. per cub. ft. 28 6 

Cost per square 41 6 

which is equivalent to 3s. 9d. per yard super. 






SPECIFICATIONS, QUANTITIES, ESTIMATING, ETC. 



287 



It is usual to charge one-third the cost of 
materials for centering of this description 
for use and waste, on the assumption that 
they may be used three times before being 
discarded, each re-use entailing, of course, 
further cutting and waste to suit the several 
positions ; on this assumption the cost of a 
yard super of centering to floor slabs 12 ft. 
high would be made up as follows 



d. 



^rd of 3s. 9d. for use and waste of 

materials 
Cost of labour and nails erecting and 

removing at 7s. 6d. per square 



1 11 



Add 15 per cent, for profit and 

establishment charges 3^ 

Cost of centering and strutting per 

yard super 2 2 

Where heights vary the timbering will not 
vary in the same ratio, for the reason that, 
while the joists and horizontal timbers' 
remain the same, the posts and braces will 
vary both in length and scantling. The 
amount of timber making up the 18 cub. ft. 
per square of centering in the foregoing 
example may be split up nearly into 7 cub. ft. 
in horizontal members and short battens, 
and 11 cub. ft. in posts and braces, costing 
respectively lls. Id. and 17s. 5d. per square, 
which is equivalent to Is. and Is. 6d. per 
yard super, ^rd of which for use and waste 
amounts to 4d. and 6d. respectively. The 
latter prices may, within limits, be taken 
for all practical purposes to vary directly 
as the height, and so charged as d. per foot of 
height ; any considerable increase in height 
will, however, necessitate an increase in 
scantlings, and will, therefore, upset this 
ratio. Beam centering or centering for any 
other purpose may be worked out in a 
similar manner ; the relative amount of 
labour will vary in each case, so will the 
proportion to be adopted for use and waste, 
as previously explained, and the value of these 
these two factors can only be ascertained 
by experience. They can never be stated 
with mathematical accuracy, because they 
will be found to vary more or less on every 
job, and the amount of labour particularly 



will vary under different foremen. It is a 
good practice to take every opportunity of 
noting on each job for future reference the 
relative amount of waste and cost of labour 
in different positions. 

It is a good exercise for the student to 
make drawings, however rough, of centering 
and strutting for different positions and 
heights, and to price them in detail in 
accordance with the foregoing principles. 

Approximate or Average Prices. 
As before stated, the prices given must be 
taken as approximate or average only. 
Prices of material and labour are continually 
changing and competitive prices vary still 
more, according to the state of the building 
trade. Builders, when trade is slack, are 
willing to cut their profit to almost vanishing 
point in order to keep their works going. 
From two recently priced bills, it is found 
that in one concrete varies from 21s. 9d. to 
23s. 6d. per yard cube, and centering to 
floor slabs is priced at a uniform rate 
throughout of 2s. per yard, which is about 
22s. 2|d. per square ; while in the other 
concrete is priced at a uniform rate of 
28s. 6d. per yard cube, and centering to floor 
slabs at 17s. per square for a height of 
12 ft., and 17s. 6d. for a height of 14 ft. It 
will be obvious from this one instance that 
any list of prices can only be taken as the 
merest guide. Moreover, the object here 
has been not to compile a price book, but 
to state the methods of measuring reinforced 
concrete work and to enunciate the principles 
on which the various prices are built up. 
In all new methods of construction it is 
essential that the quantity surveyor should 
keep himself acquainted with the broad 
principles governing their design and execu- 
tion, in order that he may measure them 
intelligently and properly separate the 
different values. The estimator must keep 
himself posted in the current prices of 
materials, and by keen observation gradually 
build up data of the amount of labour 
entailed in the different items. He must 
also make himself acquainted with the cost 
of installation and removal of plant, per- 
centage for wear and tear, etc., either for 
insertion in the preliminary bill or appor- 
tionment to the various items affected, as the 
case may be. 



Arches and Bridges 



DESIGN OF ARCHES AND BRIDGES 

THERE is probably no branch of reinforced 
concrete work that shows the advantage 
of this material more fully than that of 
arches and bridges, as it lends itself readily 
to scientific design and architectural treat- 
ment, as will be seen upon referrin